  We investigate the discrete chiral transformation of a Majorana fermion on a
torus. Depending on the boundary conditions the integration measure can change
sign. Taking this anomalous behavior into account we define a chiral order
parameter as a ratio of partition functions with differing boundary conditions.
Then the lattice realization of the Gross-Neveu model with Wilson fermions is
simulated using the recent `worm' technique on the loop gas or all-order
hopping representation of the fermions. An algorithm is formulated that
includes the Gross-Neveu interaction for N fermion species. The critical line
m_c(g) is constructed for a range of couplings at N = 6 and for N = 2, the
Thirring model, as examples.
