fix bug of repeated substitutions

Calling "kwant.continuum.discretize('A * k_x', substitutions={'A': 'A + B'})"
ends with performing substitutions twice. Fixed by removing redundant second
substitution. Fixes issue #121.

kwant/continuum/discretizer.py

@@ -78,7 +78,7 @@ def discretize(hamiltonian, discrete_coordinates=None, *, grid_spacing=1,
                                 substitutions=substitutions, verbose=verbose)
 
     return build_discretized(tb, coords, grid_spacing=grid_spacing,
-                             substitutions=substitutions, verbose=verbose)
+                             verbose=verbose)
 
 
 def discretize_symbolic(hamiltonian, discrete_coordinates=None, *,

kwant/continuum/tests/test_discretizer.py

@@ -139,6 +139,7 @@ def test_simple_derivations(commutative):
 
 
 @pytest.mark.parametrize('e_to_subs, e, subs', [
+    ('A * k_x', '(A + B) * k_x', {'A': 'A + B'}),
     ('k_x', 'k_x + k_y', {'k_x': 'k_x + k_y'}),
     ('k_x**2 + V', 'k_x**2 + V + V_0', {'V': 'V + V_0'}),
     ('k_x**2 + A + C', 'k_x**2 + B + 5', {'A': 'B + 5', 'C': 0}),
@@ -393,6 +394,19 @@ def test_numeric_functions_basic_string():
         assert +1j * p['t'] == builder[lat(1), lat(0)](None, None, **p)
 
 
+@pytest.mark.parametrize('e_to_subs, e, subs', [
+    ('A * k_x + V', '(A + B) * k_x + A + B', {'A': 'A + B', 'V': 'A + B'}),
+])
+def test_numeric_functions_with_subs(e_to_subs, e, subs):
+    p = {'A': 1, 'B': 2}
+    builder_direct = discretize(e)
+    builder_subs = discretize(e_to_subs, substitutions=subs)
+
+    lat = next(iter(builder_direct.sites()))[0]
+    assert builder_direct[lat(0)](None, **p) == builder_subs[lat(0)](None, **p)
+
+
+
 def test_numeric_functions_advance():
     hams = [
         kx**2,