function cb( error, result ) {
			if ( flg ) {
				// Prevent further processing of collection elements:
				return;
			}
			if ( error ) {
				flg = true;
				return clbk( error );
			}
			collection[ j ] = result;
			clbk();
		}
Callback invoked once a provided function finishes processing a collection element.

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