sage: Normalize sign of polynomial factors in prover

The prover, when run on recent sage versions,  failed to prove some of its
goals due to a change in sage. This commit adapts our code accordingly.
The prover passes again after this commit.

sage/group_prover.sage

@@ -177,6 +177,30 @@ class constraints:
   def __repr__(self):
     return "%s" % self
 
+def normalize_factor(p):
+  """Normalizes the sign of primitive polynomials (as returned by factor())
+
+  This function ensures that the polynomial has a positive leading coefficient.
+
+  This is necessary because recent sage versions (starting with v9.3 or v9.4,
+  we don't know) are inconsistent about the placement of the minus sign in
+  polynomial factorizations:
+  ```
+  sage: R.<ax,bx,ay,by,Az,Bz,Ai,Bi> = PolynomialRing(QQ,8,order='invlex')
+  sage: R((-2 * (bx - ax)) ^ 1).factor()
+  (-2) * (bx - ax)
+  sage: R((-2 * (bx - ax)) ^ 2).factor()
+  (4) * (-bx + ax)^2
+  sage: R((-2 * (bx - ax)) ^ 3).factor()
+  (8) * (-bx + ax)^3
+  ```
+  """
+  # Assert p is not 0 and that its non-zero coeffients are coprime.
+  # (We could just work with the primitive part p/p.content() but we want to be
+  # aware if factor() does not return a primitive part in future sage versions.)
+  assert p.content() == 1
+  # Ensure that the first non-zero coefficient is positive.
+  return p if p.lc() > 0 else -p
 
 def conflicts(R, con):
   """Check whether any of the passed non-zero assumptions is implied by the zero assumptions"""
@@ -204,10 +228,10 @@ def get_nonzero_set(R, assume):
   nonzero = set()
   for nz in map(numerator, assume.nonzero):
     for (f,n) in nz.factor():
-      nonzero.add(f)
+      nonzero.add(normalize_factor(f))
     rnz = zero.reduce(nz)
     for (f,n) in rnz.factor():
-      nonzero.add(f)
+      nonzero.add(normalize_factor(f))
   return nonzero
 
 
@@ -222,27 +246,27 @@ def prove_nonzero(R, exprs, assume):
       return (False, [exprs[expr]])
   allexprs = reduce(lambda a,b: numerator(a)*numerator(b), exprs, 1)
   for (f, n) in allexprs.factor():
-    if f not in nonzero:
+    if normalize_factor(f) not in nonzero:
       ok = False
   if ok:
     return (True, None)
   ok = True
   for (f, n) in zero.reduce(allexprs).factor():
-    if f not in nonzero:
+    if normalize_factor(f) not in nonzero:
       ok = False
   if ok:
     return (True, None)
   ok = True
   for expr in exprs:
     for (f,n) in numerator(expr).factor():
-      if f not in nonzero:
+      if normalize_factor(f) not in nonzero:
         ok = False
   if ok:
     return (True, None)
   ok = True
   for expr in exprs:
     for (f,n) in zero.reduce(numerator(expr)).factor():
-      if f not in nonzero:
+      if normalize_factor(f) not in nonzero:
         expl.add(exprs[expr])
   if expl:
     return (False, list(expl))
@@ -279,17 +303,17 @@ def describe_extra(R, assume, assumeExtra):
     if base not in zero:
       add = []
       for (f, n) in numerator(base).factor():
-        if f not in nonzero:
-          add += ["%s" % f]
+        if normalize_factor(f) not in nonzero:
+          add += ["%s" % normalize_factor(f)]
       if add:
         ret.add((" * ".join(add)) + " = 0 [%s]" % assumeExtra.zero[base])
   # Iterate over the extra nonzero expressions
   for nz in assumeExtra.nonzero:
     nzr = zeroextra.reduce(numerator(nz))
     if nzr not in zeroextra:
       for (f,n) in nzr.factor():
-        if zeroextra.reduce(f) not in nonzero:
-          ret.add("%s != 0" % zeroextra.reduce(f))
+        if normalize_factor(zeroextra.reduce(f)) not in nonzero:
+          ret.add("%s != 0" % normalize_factor(zeroextra.reduce(f)))
   return ", ".join(x for x in ret)