Demonstration of dependencies of references. Within b.c we find the
reference to x; we resolve that to a vertex which contains conjuncts
from line 1 and and line 4. We spot that line 4 is in the same (or
ancestor) lexical scope as b.c, and so we ensure that if line 5
survives, then line 4 must too, otherwise c: x will reference an
unknown field.

See also 53, 54, and 55.

-- a.cue --
a: x: 5
b: {
	a
	x: int
	c: x
}
-- out/trim --
== a.cue
a: x: 5
b: {
	a
	x: int
	c: x
}
