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Gets a Python
dict of the unbound references in
Compuations that are equal will have the same collections of unbounded
references, so it is safe to use
comp as the key for this
dict even though
a given compuation may appear in many positions in the AST.
comp: The computation building block to parse.
dict of elements where keys are the compuations in
values are a Python
set of the names of the unbound references in the
subtree of that compuation.