MT-SUBSUMPTION-CHECK is a flag or parameter.
If SAME-CONNS or T, will check whether the node about to be added is
duplicated elsewhere in the tree, and will reject it if it is. (This will use
the SAME-TAG function described below, and then do a more thorough check if the
tags match.)

If SUBSET-CONNS, will check whether the connections at the node about to be
added are a subset of those at some other node. (This is only really useful in
MT94-11, where all possible new nodes are added, breadth-first, to the tree.
It is probably too restrictive for the other mtree searches.)

If SAME-TAG will check whether the tag (an integer generated from the list of
connections) is the same as any other existing tag, and will reject it if it is.
See TAG-CONN-FN and TAG-LIST-FN. (Note that most tag functions can produce the
same tag for different matings, so this may reject connections unnecessarily.)

If NIL, will turn off subsumption checking altogether.
It belongs to subjects MTREE-TOP .
Its default value is SAME-CONNS
Its current value is SAME-CONNS.

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