function callbackBody( ch1 ) {
	if ( ch1 === 'c' ) {
		return 'return ( stdlib_complex64_real( x ) == 0.0f && stdlib_complex64_imag( x ) == 0.0f );';
	}
	if ( ch1 === 'z' ) {
		return 'return ( stdlib_complex128_real( x ) == 0.0 && stdlib_complex128_imag( x ) == 0.0 );';
	}
	if ( ch1 === 'x' ) {
		return 'return x;';
	}
	if ( ch1 === 'd' ) {
		return 'return x == 0.0;';
	}
	if ( ch1 === 'f' ) {
		return 'return x == 0.0f;';
	}
	return 'return x == 0;';
}
Returns a callback body based on an output data type.

@private
@param {string} ch1 - one-letter character abbreviation for output data type
@returns {string} callback body