function cb( error, bool ) {
			if ( flg ) {
				// Prevent further processing of collection elements:
				return;
			}
			if ( error ) {
				flg = true;
				return clbk( error );
			}
			debug( 'Collection element %d group: %s.', j, ( bool ) ? '0' : '1' );

			// Default is to return values...
			if ( opts.returns === 'indices' ) {
				if ( bool ) {
					out[ 0 ].push( j );
				} else {
					out[ 1 ].push( j );
				}
			} else if ( opts.returns === '*' ) {
				if ( bool ) {
					out[ 0 ].push( [ j, v ] );
				} else {
					out[ 1 ].push( [ j, v ] );
				}
			} else if ( bool ) {
				out[ 0 ].push( v );
			} else {
				out[ 1 ].push( v );
			}
			clbk();
		}
Callback invoked once a provided function finishes processing a collection element.

@private
@param {*} [error] - error
@param {*} [bool] - group indicator
@returns {void}