- pwp_unite(+Bindings, +Context0, -Context:list(atom=any))[private]
- merges the new Bindings with the bindings in the outer Context0,
constructing a new list of VariableName=CurrentValue bindings in
Context1. This is only used when the CurrentValue parts of the
new Bindings are known to be distinct new variables, so the
Bindings cannot possibly conflict with any existing binding in
Context0. This is O(|Bindings|.|Context0|), which is not that
efficient, but since we do not expect there to be very many
variables it doesn't matter much.
