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

			// Checking for an "own" property is necessary to guard against the edge case where an indicator function returns a group identifier which matches a method or property on the `Object` prototype.
			if ( hasOwnProp( out, group ) ) {
				out[ group ] += 1;
			} else {
				out[ group ] = 1;
			}
			clbk();
		}
Callback invoked once a provided function finishes processing a collection element.

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