ensuringArgumentIsVarOrChar
fun <C : ExecutionContext> Solve.Request<C>.ensuringArgumentIsVarOrChar(index: Int): Solve.Request<C>
Ensures the argument at index is either unbound, end_of_file, or a one-character atom, as required by the "character" argument of get_char/1,2, peek_char/1,2 and put_char/1,2.
Throws
(in_character) otherwise.