Place two diagrams (or other objects) vertically adjacent to one
another, with the first diagram above the second. Since Haskell
ignores whitespace in expressions, one can thus write
c
===
d
to place c above d. The local origin of the
resulting combined diagram is the same as the local origin of the
first. (===) is associative and has mempty as an
identity. See the documentation of beside for more information.