u64

u64 -- returns value of unit type u64s

This is a convenience feature that allows using, e.g., 'u64.sum' to
get the the monoid of (u64, infix +) instead of 'u64s.sum'.

since this u64 with no arguments is a routine and not a constructor, it
does not define a type (which would cause a name clash with u64 with one
argument).