projection maps are surjective

if the components are monoids (hence have points) and the
index has decidable equality (hence is a set)

  : compiling Utilities.v ; >Utilities.timing \time -f "%U sec, %M bytes: Utilities" coqc   -R . Ktheory -no-native-compiler -indices-matter -time  Utilities
  0.11 sec, 438730752 bytes: Utilities
  : compiling Categories.v ; >Categories.timing \time -f "%U sec, %M bytes: Categories" coqc   -R . Ktheory -no-native-compiler -indices-matter -time  Categories
  1.46 sec, 1396277248 bytes: Categories
  ./Categories.v:27646-27650: [Qed.] 0.232 secs (0.231u,0.s)
  ./Categories.v:38709-38717: [Defined.] 0.163 secs (0.162u,0.s)
  ./Categories.v:20290-20294: [Qed.] 0.112 secs (0.112u,0.s)
  ./Categories.v:27578-27645: [exact_op~(assoc~_~_~_~_~_~(Cop...] 0.051 secs (0.05u,0.001s)
  ./Categories.v:38542-38564: [apply~El.mor_equality.] 0.045 secs (0.044u,0.s)
  ./Categories.v:18147-18155: [Defined.] 0.037 secs (0.037u,0.s)
  ./Categories.v:17183-17206: [End~StandardCategories.] 0.036 secs (0.033u,0.003s)
  ./Categories.v:20202-20221: [apply~mor_equality.] 0.033 secs (0.032u,0.s)
  ./Categories.v:20250-20269: [apply~mor_equality.] 0.024 secs (0.024u,0.s)
  ./Categories.v:19752-19788: [intros~?~?~(c,~x)~(d,~y)~(f,~i...] 0.022 secs (0.022u,0.s)