We had to add NoFace/NoEdge/NoVert constructors to our half-edge
data structures, because using "Maybe HeEdge a" as the result value
of 'indirectToDirect' causes an infinite recursion, since the whole
data structure (which is cyclic and infinite) has to be evaluated
in order to know which constructor to use.
Unfortunately this makes the code quite hard to read.
TODO: add pseudo-code
