'Returns the curvature of the expression.'

@property
def curvature(self):

    if self.is_constant():
        curvature_str = s.CONSTANT
    elif self.is_affine():
        curvature_str = s.AFFINE
    elif self.is_convex():
        curvature_str = s.CONVEX
    elif self.is_concave():
        curvature_str = s.CONCAVE
    else:
        curvature_str = s.UNKNOWN
    return curvature_str
