The feature structures are supposed to be directed acyclic graphs, that is, you can't follow a path and get back to the same place. The following constraint is cyclic, and therefore illegal:

foo := basic-verb-lex &
  [ SYNSEM #synsem & 
      [ LOCAL.CAT.VAL.COMPS < #synsem > ]].

That would seem an unlikely thing for anyone to type. A more frequent cause of cycles is in diff-lists. If you are getting this error, there's a reasonably good chance that you're over-identifying the values of features that are involved in a diff-list append. Alternatively, a type and its supertype might be saying inconsistent things about the length of the diff-list. Check features such as RELS, HCONS, SLASH, QUE, and REL.

• What's a difference list, and why do we use them?

• Some of my relations/qeqs aren't showing up in the MRS for the whole parse. Why not?

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