u => (e,c)
U => (E,C)
  E => u => (e,c)
  C => (U_E[], U_C[])

u: unit (minor)
e: extent (minor, opaque)
c: content (minor, opaque)
U: Unit (major)
E: Extent (major)
C: Content (major)
U_E: Unit w/ extent semantics
U_C: Unit w/ content semantics