Trac #33858: Further parallelization for fundamental group of curves.

The computation of the fundamental group of algebraic plane curves is
done by computing the braid monodromy (which already takes advantage of
parallelization), and then computing the action of the braids over the
free group.

This second step can take non negligible time in some cases. With this
simple change, it is computed in parallel too, which can save some time.

URL: https://trac.sagemath.org/33858
Reported by: mmarco
Ticket author(s): Miguel Marco
Reviewer(s): Travis Scrimshaw

src/sage/schemes/curves/zariski_vankampen.py

@@ -1021,7 +1021,11 @@ def fundamental_group(f, simplified=True, projective=False):
     bm = braid_monodromy(f)
     n = bm[0].parent().strands()
     F = FreeGroup(n)
-    R = [x*b/x for x in F.gens() for b in bm]
+    @parallel
+    def relation(x,b):
+        return x*b/x
+    relations = list(relation([(x,b) for x in F.gens() for b in bm]))
+    R = [r[1] for r in relations]
     if projective:
         R.append(prod(F.gens()))
     G = F/R