Given a codepoint c, return the codepoint of the corresponding lower-case character, if any; otherwise (if there is no lower-case variant, or if c is not a valid codepoint) return c.
See Implementation
Given a codepoint c, return the codepoint of the corresponding lower-case character, if any; otherwise (if there is no lower-case variant, or if c is not a valid codepoint) return c.