LinearChildren​(AngouriMath.​Entity)

1 + (x - a/2) + b - 4

{ 1, x, (-1) * a / 2, b, (-1) * 4 }

Cached on the node it is asked of, since a node is immutable and the flattening
is asked for many times over: every rule that reads a sum as a list reads it
again for each candidate. A nested sum's list is built from its parts' lists,
which are cached on those parts, so the memory is linear in the tree and no
enumeration is repeated. https://github.com/asc-community/AngouriMath/issues/224