acme-zero and acme-one

Cabal package dependencies form a commutative monoid with an absorbing element. They satisfy the following axioms:

• Associativity

  For all dependencies a, b and c,

  (a , b) , c = a , (b , c)
  

  In fact, the Cabal syntax does not even allow you to use the parentheses for this reason.

• Commutativity

  For all dependencies a and b,

  a , b = b , a
  

• Identity element

  For all dependencies a,

  acme-one , a = a
  a , acme-one = a
  

  Depending on acme-one has no additional effect.

• Absorbing element

  For all dependencies a,

  acme-zero , a = acme-zero
  a , acme-zero = acme-zero
  

  Depending on acme-zero in addition has the same effect as only depending on acme-zero.