add doc

docs/lib2x-flat.md

@@ -0,0 +1,41 @@
+# flat
+
+returns a new list with all sub-list elements concatenated into it recursively up to the specified depth.
+
+**Since:** 2.3
+
+## Parameters
+
+- `lt` : The original list.
+- `depth` : Defaults to 1. The depth level specifying how deep a nested list should be flattened. 
+
+## Examples
+
+	use <util/flat.scad>;
+
+	vt = [[[[1, 2], [3, 4]], [[5, 6], [7, 8]]]];
+
+	assert(
+		flat([1, 2, [3, 4]]) == 
+			[1, 2, 3, 4]
+	);
+
+	assert(
+		flat([[1, 2], [3, 4]]) == 
+			[1, 2, 3, 4]
+	);
+
+	assert(
+		flat([[[[1, 2], [3, 4]], [[5, 6], [7, 8]]]]) == 
+			[[[1, 2], [3, 4]], [[5, 6], [7, 8]]]
+	);
+
+	assert(
+		flat([[[[1, 2], [3, 4]], [[5, 6], [7, 8]]]], 2) == 
+			[[1, 2], [3, 4], [5, 6], [7, 8]]
+	);
+
+	assert(
+		flat([[[[1, 2], [3, 4]], [[5, 6], [7, 8]]]], 3) == 
+			[1, 2, 3, 4, 5, 6, 7, 8]
+	);