function callbackBody( ch1, ch2 ) {
	if ( isComplexChar( ch1 ) || isComplexChar( ch2 ) || ch1 !== ch2 ) {
		return '// ...';
	}
	return 'return x;';
}
Returns a callback body based on callback input and output data types.

@private
@param {string} ch1 - one-letter character abbreviation for input argument
@param {string} ch2 - one-letter character abbreviation for output value
@returns {string} callback body