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docs/lib2-sub_str.md

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+# sub_str
+
+Returns a new string that is a substring of the given string.
+
+## Parameters
+
+- `t` : The original string.
+- `begin` : The beginning index, inclusive.
+- `end` : The ending index, exclusive. If it's omitted, the substring begins with the character at the specified `begin` and extends to the end of the original string.
+
+## Examples
+
+    include <util/sub_str.scad>;
+    
+	echo(sub_str("helloworld", 0, 5)); // ECHO: "hello"
+	echo(sub_str("helloworld", 5));    // ECHO: "world"