variables are constructed explicitly rather than using the capitalized symbol
convention
neg matchers can declare variables, although the variables are only used for
unification if they're used more than once within the same template, and do
not escape the scope of the matcher
assert is called filter
assuming clauses are not implemented yet
queries are not implemented but may be in the future
maybe/optional matcher is not implemented since it's unclear at the moment
whether it actually serves a purpose
overlays are not implemented and are not going to be, since the immutable
nature of the language makes almost the entire feature redundant
self-invalidation detection is not implemented and likely isn't going to be,
since it is computationally quite expensive while still detecting only a small
set of possible contradictions, which is likely impossible to solve in a
general case anyway since it seems to boil down to the halting problem; so be
aware that your rules may create infinite loops when you generate facts that
invalidate their own creation