diff --git a/api/src/main/groovy/com/jessebrault/ssg/SiteSpec.groovy b/api/src/main/groovy/com/jessebrault/ssg/SiteSpec.groovy
index 10c248d..3b5f0e6 100644
--- a/api/src/main/groovy/com/jessebrault/ssg/SiteSpec.groovy
+++ b/api/src/main/groovy/com/jessebrault/ssg/SiteSpec.groovy
@@ -1,5 +1,6 @@
 package com.jessebrault.ssg
 
+import com.jessebrault.ssg.util.Monoid
 import groovy.transform.EqualsAndHashCode
 import groovy.transform.NullCheck
 import groovy.transform.TupleConstructor
@@ -9,6 +10,11 @@ import groovy.transform.TupleConstructor
 @EqualsAndHashCode
 final class SiteSpec {
 
+    static final Monoid<SiteSpec> defaultSemiGroup = new Monoid<>(
+            SiteSpec::concat,
+            SiteSpec::getBlank
+    )
+
     static SiteSpec getBlank() {
         new SiteSpec('', '')
     }
diff --git a/api/src/main/groovy/com/jessebrault/ssg/buildscript/domain/MutableSiteSpec.groovy b/api/src/main/groovy/com/jessebrault/ssg/buildscript/domain/MutableSiteSpec.groovy
new file mode 100644
index 0000000..8c738ad
--- /dev/null
+++ b/api/src/main/groovy/com/jessebrault/ssg/buildscript/domain/MutableSiteSpec.groovy
@@ -0,0 +1,29 @@
+package com.jessebrault.ssg.buildscript.domain
+
+import com.jessebrault.ssg.property.Properties
+import com.jessebrault.ssg.property.Property
+import com.jessebrault.ssg.util.Monoid
+
+final class MutableSiteSpec {
+
+    private static final Monoid<String> nameAndBaseUrlMonoid = new Monoid<>(
+            { s0, s1 -> s1 },
+            { '' }
+    )
+
+    static final Monoid<MutableSiteSpec> MONOID = new Monoid<>(
+            MutableSiteSpec::concat,
+            MutableSiteSpec::new
+    )
+
+    private static MutableSiteSpec concat(MutableSiteSpec ms0, MutableSiteSpec ms1) {
+        new MutableSiteSpec().tap {
+            name.set(ms1.name.get())
+            baseUrl.set(ms1.baseUrl.get())
+        }
+    }
+
+    final Property<String> name = Properties.get(nameAndBaseUrlMonoid)
+    final Property<String> baseUrl = Properties.get(nameAndBaseUrlMonoid)
+
+}
diff --git a/api/src/main/groovy/com/jessebrault/ssg/property/CollectionProperty.groovy b/api/src/main/groovy/com/jessebrault/ssg/property/CollectionProperty.groovy
deleted file mode 100644
index aefbe29..0000000
--- a/api/src/main/groovy/com/jessebrault/ssg/property/CollectionProperty.groovy
+++ /dev/null
@@ -1,5 +0,0 @@
-package com.jessebrault.ssg.property
-
-interface CollectionProperty<T> extends Property<T> {
-    // TODO
-}
\ No newline at end of file
diff --git a/api/src/main/groovy/com/jessebrault/ssg/property/MapProperty.groovy b/api/src/main/groovy/com/jessebrault/ssg/property/MapProperty.groovy
deleted file mode 100644
index 4a0763e..0000000
--- a/api/src/main/groovy/com/jessebrault/ssg/property/MapProperty.groovy
+++ /dev/null
@@ -1,7 +0,0 @@
-package com.jessebrault.ssg.property
-
-interface MapProperty<K, V> extends Property<Map<K, V>> {
-    V get(K key)
-    void put(K key, V value)
-    void putAll(Map<? extends K, ? extends V> map)
-}
diff --git a/api/src/main/groovy/com/jessebrault/ssg/property/Properties.groovy b/api/src/main/groovy/com/jessebrault/ssg/property/Properties.groovy
index f4f8811..fb95e30 100644
--- a/api/src/main/groovy/com/jessebrault/ssg/property/Properties.groovy
+++ b/api/src/main/groovy/com/jessebrault/ssg/property/Properties.groovy
@@ -1,32 +1,19 @@
 package com.jessebrault.ssg.property
 
+import com.jessebrault.ssg.util.Monoid
+
 final class Properties {
 
-    static <T> Property<T> get() {
-        new SimpleProperty<>()
+    static <T> Property<T> get(Monoid<T> semiGroup) {
+        new SimpleProperty<>(semiGroup)
     }
 
-    static <T> Property<T> get(T convention) {
-        new SimpleProperty<>(convention)
+    static <T> Property<T> get(Monoid<T> semiGroup, T convention) {
+        new SimpleProperty<>(semiGroup, convention)
     }
 
-    static <T> Property<T> get(T convention, T t) {
-        new SimpleProperty<>(convention, t)
-    }
-
-    static <K, V> MapProperty<K, V> getMap() {
-        new SimpleMapProperty<K, V>()
-    }
-
-    static <K, V> MapProperty<K, V> getMap(Map<? extends K, ? extends V> convention) {
-        new SimpleMapProperty<K, V>(convention)
-    }
-
-    static <K, V> MapProperty<K, V> getMap(
-            Map<? extends K, ? extends V> convention,
-            Map<? extends K, ? extends V> value
-    ) {
-        new SimpleMapProperty<K, V>(convention, value)
+    static <T> Property<T> get(Monoid<T> semiGroup, T convention, T t) {
+        new SimpleProperty<>(semiGroup, convention, t)
     }
 
     private Properties() {}
diff --git a/api/src/main/groovy/com/jessebrault/ssg/property/Property.groovy b/api/src/main/groovy/com/jessebrault/ssg/property/Property.groovy
index 48574d1..654ae78 100644
--- a/api/src/main/groovy/com/jessebrault/ssg/property/Property.groovy
+++ b/api/src/main/groovy/com/jessebrault/ssg/property/Property.groovy
@@ -1,38 +1,19 @@
 package com.jessebrault.ssg.property
 
-import com.jessebrault.ssg.provider.Provider
-
-import java.util.function.Function
 import java.util.function.UnaryOperator
 
-interface Property<T> extends Provider<T> {
-
+interface Property<T> {
+    T get()
     void set(T t)
-    void set(Provider<T> provider)
     void unset()
 
-    default void leftShift(T t) {
-        this.set(t)
-    }
-
-    default void leftShift(Provider<T> provider) {
-        this.set(provider)
-    }
-
     T getConvention()
     void setConvention(T t)
-    void setConvention(Provider<T> provider)
     void unsetConvention()
 
-    void map(UnaryOperator<T> operator)
-    void flatMap(Function<T, Provider<T>> function)
-
-    default void rightShift(UnaryOperator<T> operator) {
-        this.map(operator)
-    }
-
-    default void rightShift(Function<T, Provider<T>> function) {
-        this.flatMap(function)
-    }
-
+    void map(UnaryOperator<T> mapper)
+    void merge(
+            @DelegatesTo(type = 'T', strategy = Closure.DELEGATE_FIRST)
+            Closure<?> configurator
+    )
 }
\ No newline at end of file
diff --git a/api/src/main/groovy/com/jessebrault/ssg/property/SimpleMapProperty.groovy b/api/src/main/groovy/com/jessebrault/ssg/property/SimpleMapProperty.groovy
deleted file mode 100644
index 7f348b3..0000000
--- a/api/src/main/groovy/com/jessebrault/ssg/property/SimpleMapProperty.groovy
+++ /dev/null
@@ -1,122 +0,0 @@
-package com.jessebrault.ssg.property
-
-import com.jessebrault.ssg.provider.CollectionProvider
-import com.jessebrault.ssg.provider.Provider
-import groovy.transform.EqualsAndHashCode
-import groovy.transform.NullCheck
-import groovy.transform.PackageScope
-
-import java.util.function.Function
-import java.util.function.UnaryOperator
-
-@PackageScope
-@NullCheck
-@EqualsAndHashCode(includeFields = true)
-final class SimpleMapProperty<K, V> implements MapProperty<K, V> {
-
-    private final Map<K, V> convention = [:]
-    private final Map<K, V> m = [:]
-
-    SimpleMapProperty(
-            Map<? extends K, ? extends V> convention,
-            Map<? extends K, ? extends V> m
-    ) {
-        this.convention.putAll(convention)
-        this.m.putAll(m)
-    }
-
-    SimpleMapProperty(Map<? extends K, ? extends V> convention) {
-        this(convention, [:] as Map<K, V>)
-    }
-
-    SimpleMapProperty() {}
-
-    @Override
-    V get(K key) {
-        if (m[key] != null) {
-            m[key]
-        } else if (convention[key] != null) {
-            convention[key]
-        } else {
-            throw new NullPointerException("no such key: ${ key }")
-        }
-    }
-
-    @Override
-    void put(K key, V value) {
-        this.m.put(key, value)
-    }
-
-    @Override
-    void putAll(Map<? extends K, ? extends V> map) {
-        this.m.putAll(map)
-    }
-
-    @Override
-    void set(Map<K, V> kvMap) {
-        this.m.clear()
-        this.m.putAll(kvMap)
-    }
-
-    @Override
-    void set(Provider<Map<K, V>> provider) {
-        // TODO
-    }
-
-    @Override
-    void unset() {
-        this.m.clear()
-    }
-
-    @Override
-    Map<K, V> getConvention() {
-        this.convention
-    }
-
-    @Override
-    void setConvention(Map<K, V> kvMap) {
-        this.convention.clear()
-        this.convention.putAll(kvMap)
-    }
-
-    @Override
-    void setConvention(Provider<Map<K, V>> provider) {
-        // TODO
-    }
-
-    @Override
-    void unsetConvention() {
-        this.convention.clear()
-    }
-
-    @Override
-    void map(UnaryOperator<Map<K, V>> operator) {
-        this.m.putAll(operator.apply(this.convention + this.m))
-    }
-
-    @Override
-    void flatMap(Function<Map<K, V>, Provider<Map<K, V>>> function) {
-        // TODO
-    }
-
-    @Override
-    Map<K, V> provide() {
-        this.convention + this.m
-    }
-
-    @Override
-    CollectionProvider<Map<K, V>> plus(Provider<Map<K, V>> other) {
-        // TODO
-    }
-
-    @Override
-    CollectionProvider<Map<K, V>> asType(Class<CollectionProvider> collectionProviderClass) {
-        // TODO
-    }
-
-    @Override
-    boolean isEmpty() {
-        return false
-    }
-
-}
diff --git a/api/src/main/groovy/com/jessebrault/ssg/property/SimpleProperty.groovy b/api/src/main/groovy/com/jessebrault/ssg/property/SimpleProperty.groovy
index 34b4ae5..792daeb 100644
--- a/api/src/main/groovy/com/jessebrault/ssg/property/SimpleProperty.groovy
+++ b/api/src/main/groovy/com/jessebrault/ssg/property/SimpleProperty.groovy
@@ -1,93 +1,85 @@
 package com.jessebrault.ssg.property
 
-import com.jessebrault.ssg.provider.AbstractProvider
-import com.jessebrault.ssg.provider.Provider
-import com.jessebrault.ssg.provider.Providers
+import com.jessebrault.ssg.util.Monoid
 import groovy.transform.EqualsAndHashCode
 import groovy.transform.NullCheck
 import groovy.transform.PackageScope
 
-import java.util.function.Function
 import java.util.function.UnaryOperator
 
+import static java.util.Objects.requireNonNull
+
 @PackageScope
-@NullCheck
+@NullCheck(includeGenerated = true)
 @EqualsAndHashCode(includeFields = true)
-final class SimpleProperty<T> extends AbstractProvider<T> implements Property<T> {
+final class SimpleProperty<T> implements Property<T> {
 
-    private Provider<T> convention = Providers.getEmpty()
-    private Provider<T> tProvider = Providers.getEmpty()
+    private final Monoid<T> monoid
 
-    SimpleProperty(T convention) {
-        this.convention = Providers.of(convention)
+    private T t
+    private T convention
+
+    SimpleProperty(Monoid<T> monoid) {
+        this.monoid = monoid
+        this.convention = this.monoid.empty.get()
     }
 
-    SimpleProperty(T convention, T t) {
-        this.convention = Providers.of(convention)
-        this.tProvider = Providers.of(t)
+    SimpleProperty(Monoid<T> monoid, T convention) {
+        this.monoid = monoid
+        this.convention = convention
     }
 
-    SimpleProperty() {}
+    SimpleProperty(Monoid<T> monoid, T convention, T t) {
+        this.monoid = monoid
+        this.t = t
+        this.convention = convention
+    }
 
     @Override
-    T provide() {
-        this.tProvider.present ? this.tProvider.provide() : this.convention.provide()
+    T get() {
+        this.t != null ? this.t : requireNonNull(this.convention)
     }
 
     @Override
     void set(T t) {
-        this.tProvider = Providers.of(t)
-    }
-
-    @Override
-    void set(Provider<T> provider) {
-        this.tProvider = provider
+        this.t = t
     }
 
     @Override
     void unset() {
-        this.tProvider = Providers.getEmpty()
+        this.t = null
     }
 
     @Override
     T getConvention() {
-        this.convention.provide()
+        requireNonNull(this.convention)
     }
 
     @Override
     void setConvention(T t) {
-        this.convention = Providers.of(t)
-    }
-
-    @Override
-    void setConvention(Provider<T> provider) {
-        this.convention = provider
+        this.convention = t
     }
 
     @Override
     void unsetConvention() {
-        this.convention = Providers.getEmpty()
+        this.t = null
     }
 
     @Override
-    void map(UnaryOperator<T> operator) {
-        def oldTProvider = this.tProvider
-        this.tProvider = Providers.fromSupplier {
-            operator.apply(oldTProvider.provide())
-        }
+    void map(UnaryOperator<T> mapper) {
+        this.t = requireNonNull(mapper.apply(this.t))
     }
 
     @Override
-    void flatMap(Function<T, Provider<T>> function) {
-        def oldTProvider = this.tProvider
-        this.tProvider = Providers.fromSupplier {
-            function.apply(oldTProvider.provide()).provide()
-        }
-    }
-
-    @Override
-    String toString() {
-        "SimpleProperty(convention: ${ this.convention }, tProvider: ${ this.tProvider })"
+    void merge(
+            @DelegatesTo(type = 'T', strategy = Closure.DELEGATE_FIRST)
+            Closure<?> configurator
+    ) {
+        def d = requireNonNull(this.monoid.empty.get())
+        configurator.delegate = d
+        configurator.resolveStrategy = Closure.DELEGATE_FIRST
+        configurator()
+        this.t = requireNonNull(this.monoid.concat.apply(this.t != null ? this.t : this.monoid.empty.get(), d))
     }
 
 }
diff --git a/api/src/main/groovy/com/jessebrault/ssg/util/Monoid.groovy b/api/src/main/groovy/com/jessebrault/ssg/util/Monoid.groovy
new file mode 100644
index 0000000..71becf2
--- /dev/null
+++ b/api/src/main/groovy/com/jessebrault/ssg/util/Monoid.groovy
@@ -0,0 +1,16 @@
+package com.jessebrault.ssg.util
+
+import groovy.transform.EqualsAndHashCode
+import groovy.transform.NullCheck
+import groovy.transform.TupleConstructor
+
+import java.util.function.BinaryOperator
+import java.util.function.Supplier
+
+@TupleConstructor(defaults = false)
+@NullCheck(includeGenerated = true)
+@EqualsAndHashCode
+final class Monoid<T> {
+    final BinaryOperator<T> concat
+    final Supplier<T> empty
+}
diff --git a/api/src/test/groovy/com/jessebrault/ssg/property/SimplePropertyTests.groovy b/api/src/test/groovy/com/jessebrault/ssg/property/SimplePropertyTests.groovy
new file mode 100644
index 0000000..64729f9
--- /dev/null
+++ b/api/src/test/groovy/com/jessebrault/ssg/property/SimplePropertyTests.groovy
@@ -0,0 +1,24 @@
+package com.jessebrault.ssg.property
+
+import com.jessebrault.ssg.buildscript.domain.MutableSiteSpec
+import org.junit.jupiter.api.Test
+
+import static org.junit.jupiter.api.Assertions.assertEquals
+
+final class SimplePropertyTests {
+
+    @Test
+    void merge() {
+        def p = Properties.get(MutableSiteSpec.MONOID)
+        p.merge {
+            name.set('Hello')
+        }
+        p.map {
+            it.name.map { it + ', World!' }
+            it
+        }
+        def ms = p.get()
+        assertEquals('Hello, World!', ms.name.get())
+    }
+
+}
diff --git a/sketchingBaseBuild.groovy b/sketchingBaseBuild.groovy
index ec0ea15..f52faa5 100644
--- a/sketchingBaseBuild.groovy
+++ b/sketchingBaseBuild.groovy
@@ -1,16 +1,17 @@
 abstractBuild(name: 'redDogAll', extends: 'default') {
-    siteSpec.merge {
+    // siteSpec(Closure) is short for siteSpec.merge(Closure)
+    siteSpec {
         name = 'Red Dog Ensemble'
         baseUrl = 'https://reddogensemble.com'
     }
 
-    globals.merge {
+    globals {
         greeting = 'Say hello to good music!'
     }
 
     sources { types ->
         models.map { acc ->
-            old + someMethodThatGetsEventModels()
+            acc + someMethodThatGetsEventModels()
         }
     }
 
@@ -26,11 +27,11 @@ abstractBuild(name: 'redDogAll', extends: 'default') {
 }
 
 build(name: 'preview', extends: 'redDogAll') {
-    siteSpec.merge {
-        baseUrl += '/preview' // if possible
+    siteSpec { base ->
+        baseUrl = base.baseUrl + '/preview' // if possible
     }
 
-    globals.merge {
+    globals {
         greeting = 'Hello from preview!'
     }
 }
\ No newline at end of file