In Arithmetic.m,
  add(P,Q,point0: affine_patch:=point0`numbering.0)
  is wrong in level 2 (but diff_add works in level 2).

We require a version of magma that supports AssociativeArray().
  Note: magma-2.15.* has a bug with associative array:
  a:=AssociativeArray();
  a["foo"]:=AssociativeArray();
  a["foo","bar"]:="baz";
  yields a segfault.
  This is corrected in magma-2.16.
  In the code, we try to be compatible with magma-2.15 by using the following
  variant:
  a:=AssociativeArray();
  b:=AssociativeArray();
  b["bar"]:="baz";
  a["foo"]:=b;