Why is ID 4 kept when B and A are both in other features?

Have a look at the ListBasedFeatureMerger, can be what you are looking for.

@egomm It's because they BOTH must be in another feature. A is in ID=3 and B is in ID=1, but there's no feature containing both values.

I did but can't figure out how to make it do what I want.

I think python is going to be the solution here, are there ever going to be cases where the lists are identical?

This might get you part way there, it would output the ID of any feature where all the list values are present in another single feature. You could then use a featuremerger with the original features to only keep features that are unmerged (if features can have identical lists it won't handle them correctly!)

import fme
import fmeobjects

class FeatureProcessor(object):
    def __init__(self):
        self.trips=)]
        
    def input(self,feature):
        id = feature.getAttribute('ID')
        trip = feature.getAttribute('TIPS{}.trip')
        for i in trip:
            self.trips.append((id, i))
                

    def close(self):
        ids = list(set(ÂiÂ0] for i in self.trips]))
        discard =  ]
        for x in ids:
            list1 = lis1] for i in self.trips if i 0]==x]
            for y in ids:      
                if x==y:
                    pass
                else:
                    list2 =Âi[1] for i in self.trips if is0]==y]
                    result = all(elem in list1 for elem in list2)
                    if result:
                        discard.append(y)
        for val in list(set(discard)):
            newFeature = fmeobjects.FMEFeature()
            newFeature.setAttribute('ID',val)
            self.pyoutput(newFeature)

discard_list_in_list.fmw

Disclaimer: python people might be able to suggest a better method