function cb( error, value ) {
			if ( flg ) {
				// Prevent further processing of properties:
				return;
			}
			if ( error ) {
				flg = true;
				return clbk( error );
			}
			debug( 'Transform result => %s: %s', key, JSON.stringify( value ) );
			out[ key ] = value;
			clbk();
		}
Callback invoked once a provided function finishes transforming a property value.

@private
@param {*} [error] - error
@param {*} [value] - transformed value
@returns {void}