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+////
+/// Property trees.
+///
+/// Property trees allow you to organize properties in a tree structure (internally nested maps).
+/// The intended use is to store all your properties at the beginning and for the rest of the
+/// stylesheet you just get them.
+///
+/// @group Property trees
+///
+/// @access public
+////
+
+///
+/// The maximum depth of resolved iro-prop-ref() references.
+///
+/// @type number
+///
+$iro-props-native-assing-max-depth: 2 !default;
+
+///
+/// Indicate if property names must start with two dashes (--).
+/// This is required if property trees are also used for native CSS custom properties.
+///
+/// @type bool
+///
+$iro-props-enforce-double-dashes: true !default;
+
+///
+/// Default tree name to use if no name is specified.
+///
+/// @type string
+///
+$iro-props-default-tree: 'default' !default;
+
+///
+/// List of all created property trees.
+///
+/// @type list
+///
+/// @access private
+///
+$iro-props-trees: ();
+
+/// 
+/// Save a property tree. If a tree with the sane name already exists, the trees
+/// will be merged.
+///
+/// @param {map}    $map                            - Map containing properties
+/// @param {string} $tree  [$iro-props-default-tree] - ID the map is saved as
+/// @param {bool}   $merge [false]                  - If a tree named $tree already exists and this value is set to true, they will be merged. Otherwise an error will be emitted.
+///
+@mixin iro-props-save($map, $tree: $iro-props-default-tree, $merge: false) {
+    $noop: iro-props-save($map, $tree, $merge);
+}
+
+/// 
+/// Save a property tree.
+///
+/// @param {map}    $map                            - Map containing properties
+/// @param {string} $tree  [$iro-props-default-tree] - ID the map is saved as
+/// @param {bool}   $merge [false]                  - If a tree named $tree already exists and this value is set to true, they will be merged. Otherwise an error will be emitted.
+///
+@function iro-props-save($map, $tree: $iro-props-default-tree, $merge: false) {
+    $prop-map: null;
+
+    @if $iro-props-enforce-double-dashes {
+        @if not iro-props-validate($map) {
+            @error 'Property tree keys must start with two dashes (--). If you don\'t use property trees for native CSS custom properties, set $iro-props-enforce-double-dashes to false.';
+        }
+    }
+
+    @if map-has-key($iro-props-trees, $tree) {
+        @if $merge {
+            $map: iro-map-merge-recursive(map-get($iro-props-trees, $tree), $map);
+        } @else {
+            @error 'Property tree #{inspect($tree)} does already exist.';
+        }
+    }
+
+    $iro-props-trees: map-merge($iro-props-trees, ($tree: $map)) !global;
+
+    @return null;
+}
+
+/// 
+/// Delete a property tree.
+///
+/// @param {string} $tree [$iro-props-default-tree] - ID of the tree to be deleted
+///
+@mixin iro-props-delete($tree: $iro-props-default-tree) {
+    $noop: iro-props-delete($tree);
+}
+
+/// 
+/// Unset a property tree.
+///
+/// @param {string} $tree [$iro-props-default-tree] - ID of the tree to be deleted
+///
+/// @throw If the property tree does not exist
+///
+@function iro-props-delete($tree: $iro-props-default-tree) {
+    @if not map-has-key($iro-props-trees, $tree) {
+        @error 'Property tree "#{inspect($tree)}" does not exist.';
+    }
+
+    $iro-props-trees: map-remove($iro-props-trees, $tree) !global;
+
+    @return null;
+}
+
+/// 
+/// Access a whole property or a subsection (i.e. value) of it.
+///
+/// @param {string | list} $key     [null]                   - Key of the property to read. If this is a list of keys, the map will be traversed in that order.
+/// @param {string}        $tree    [$iro-props-default-tree] - ID of the property tree to use
+/// @param {any}           $default [null]                   - Default value to return of no match was found. If null, this function will throw an error instead.
+///
+/// @return {any} Value assigned to property or $default
+///
+/// @throw If there was no match for $key and $default is null
+///
+@function iro-props-get($key: (), $tree: $iro-props-default-tree, $default: null) {
+    @if not map-has-key($iro-props-trees, $tree) {
+        @error 'Unknown tree "#{$tree}".';
+    }
+
+    $result: map-get($iro-props-trees, $tree);
+
+    @if type-of($key) == list {
+        $stop: false;
+
+        @each $k in $key {
+            @if map-has-key($result, $k) and not $stop {
+                $result: map-get($result, $k);
+
+                @if type-of($result) == list and nth($result, 1) == 'iro-prop-ref' {
+                    @if length($result) == 2 {
+                        $result: iro-props-get($tree: nth($result, 2));
+                    } @else {
+                        $result: iro-props-get(nth($result, 3), nth($result, 2));
+                    }
+                }
+            } @else {
+                $stop: true;
+            }
+        }
+
+        @if $stop {
+            $result: null;
+        }
+    } @else {
+        $result: map-get($result, $key);
+
+        @if type-of($result) == list and nth($result, 1) == 'iro-prop-ref' {
+            @if length($result) == 2 {
+                $result: iro-props-get($tree: nth($result, 2));
+            } @else {
+                $result: iro-props-get(nth($result, 3), nth($result, 2));
+            }
+        }
+    }
+
+    @if $result == null {
+        @if $default == null {
+            @error '"#{$key}" is null.';
+        } @else {
+            @return $default;
+        }
+    }
+
+    @return $result;
+}
+
+/// 
+/// Generate a var() function call to get native CSS custom property.
+///
+/// @param {string | list} $key            - Key of the property to read. If this is a list of keys, the map will be traversed in that order.
+/// @param {string | null} $tree    [null] - Optional tree to check if the property actually exists.
+/// @param {any}           $default [null] - Default value to return of no match was found.
+///
+/// @return {string} var()
+///
+@function iro-props-get-native($key, $tree: null, $default: null) {
+    @if $tree != null {
+        $noop: iro-props-get($key, $tree, $default);
+    }
+
+    $native-var: '';
+
+    @if type-of($key) == list {
+        @each $subkey in $key {
+            $native-var: $native-var + $subkey;
+        }
+    } @else {
+        $native-var: $key;
+    }
+
+    @if $default == null {
+        @return var(#{$native-var});
+    } @else {
+        @return var(#{$native-var}, #{$default});
+    }
+}
+
+/// 
+/// Generate assignments for native CSS custom properties with the values from the specified tree.
+///
+/// @param {string} $tree [$iro-props-default-tree] - ID of the property tree to use
+/// @param {string} $root [()]                     - Sub-tree to use for assignment
+///
+@mixin iro-props-assign-native($tree: $iro-props-default-tree, $root: (), $skip: ()) {
+    $map: iro-props-get($root, $tree);
+    $map: map-remove($map, $skip...);
+
+    @include iro-props-assign-native-internal($map);
+}
+
+///
+/// @access private
+///
+@mixin iro-props-assign-native-internal($map, $prefix: '', $ref-depth: $iro-props-native-assing-max-depth) {
+    @each $key, $value in $map {
+        $rd: $ref-depth;
+        @if type-of($value) == list and nth($value, 1) == 'iro-prop-ref' {
+            @if $ref-depth != 0 {
+                $rd: $rd - 1;
+                @if length($value) == 2 {
+                    $value: iro-props-get($tree: nth($value, 2));
+                } @else {
+                    $value: iro-props-get(nth($value, 3), nth($value, 2));
+                }
+            } @else {
+                $value: null;
+            }
+        }
+        @if type-of($value) != map {
+            #{$prefix + $key}: #{$value};
+        } @else {
+            @include iro-props-assign-native-internal($value, $prefix + $key, $rd);
+        }
+    }
+}
+
+/// 
+/// Validate property names.
+///
+/// @access private
+///
+@function iro-props-validate($map) {
+    @each $key, $value in $map {
+        @if str-index($key, '--') != 1 {
+            @return false;
+        }
+
+        @if type-of($value) == map {
+            @if not iro-props-validate($value) {
+                @return false;
+            }
+        }
+    }
+
+    @return true;
+}
+
+/// 
+/// Generate a reference to another tree. Dereferencing is lazy, so you may specify a tree that hasn't been created yet.
+///
+/// @param {string}        $tree [$iro-props-default-tree] - ID of the property tree to use
+/// @param {string | list} $key                           - Key of the property to read. If this is a list of keys, the map will be traversed in that order.
+///
+/// @return {list} A special list that let's Ignis know that this is a lazy value.
+///
+/// @throw If there was no match for $key and $default is null
+///
+@function iro-props-ref($tree: $iro-props-default-tree, $key: null) {
+    @if $key == null {
+        @return ('iro-prop-ref' $tree);
+    } @else {
+        @return ('iro-prop-ref' $tree $key);
+    }
+}