Save some expression into a variable.
The operand is both emitted and traversed only once: the walk runs
inside the builder's once() cache, which stores the resulting value,
so every later reference returns the cached variable without re-walking
the subtree. Memoizing inside mem is therefore safe for deep, heavily
shared expression trees — without this, a node referenced N times is
walked N times and a deep tree assembles in time exponential in its
depth.
Staticval
Get the fourth component of vector