def Z2_to_sign(f):
"""
Helper function turning the result of get_all_obstruction_classes
which uses the type field_p into +1, -1 signs.
>>> Z2_to_sign(field_p(2)(0))
1
>>> Z2_to_sign(field_p(2)(1))
-1
"""
if f == field_p(2)(0):
return + 1
elif f == field_p(2)(1):
return - 1
raise Exception, "Z2_to_sign expects field_p(2)(0) or field_p(2)(1)"
def get_all_obstruction_classes(t):
"""
Computes all classes in H^2(M, partial M; Z/2)
"""
return cohomology_2_rel_boundary_classes(t, field_p(2))
