Every capture an action receives bundles the raw parsed structure with
a callable to evaluate it: node is always available, unevaluated --
what quote returns directly. eval is what an action calls when it
actually wants a child's value, with an explicit context it controls.
Never evaluated automatically -- that's the whole point: a special
form like if or quote decides whether and when to call eval
at all, which is exactly what lets an untaken if branch, or quote's
own argument, go unevaluated (and unbound-symbol/nonterminating errors
inside them stay latent) instead of always eagerly running.