Braid Group construction misses some relations

[mmarco]

Braid Groups are constructed over a finitely presented group, but not all relations are present:

sage: B=BraidGroup('a,b,c')
sage: B
Braid group on 4 strands
sage: B.relations()
(a*b*a*b^-1*a^-1*b^-1,)

[tscrim]

The if statement is always true since range(a, b) is [a, a+1, ..., b-2, b-1] (or you can think of it as the integer points in th internal [a, b)). So I think this should work:

for i in range(1, n):
    rels.append(free_group([i, i+1, i, -i-1, -i, -i-1]))
    for j in range(i+2, n+1):
        rels.append(free_group([i, j, -i, -j]))

[mmarco]

You are right. Here is a new patch.

[tscrim]

Could you add a proper commit message? Once that's done, you can set this to positive review on my behalf. Thanks!