/* F_ab(a, i, b, j, "x") is a common pattern in all vertex evaluators. */
static ex F_ab(int a, int i, int b, int j, const symbol &x)
{
/* F_ab(a, i, b, j, "x") is a common pattern in all vertex evaluators. */
static ex F_ab(int a, int i, int b, int j, const symbol &x)
{