Mine

macro replace(s, find, replacement) {
	// replace arg2 with arg3 in arg1; store result in arg4
	// NOTE: ARG2 and ARG3 *must*  be exactly one byte!
	Repl_dst=;	// clear any old value
	for (Repl_field=1; ${LEN(${Repl_dst})} <= ${LEN(${s})}; Repl_field=${Repl_field} + 1) {
		Repl_new=${CUT(s,${find},${Repl_field})};
		Repl_dst=${Repl_dst}${replacement}${Repl_new};
	};
	Repl_var=REPLACED;
	if ("${ARG4}" != "")
		Repl_var=${ARG4};
	${Repl_var}=${Repl_dst:${LEN(${replacement})}};
};