[Fix] Printing of subtraction

Subtraction is realised in SymPy with a multiplication of -1 and an addition. Since the -1 is casted in our typing system, it remains in the printer. This leads to many unnecessary number prints. We can avoid this by not casting the negative one

pystencils_tests/test_printing.py

 import pytest
+import sympy as sp
 
 import pystencils
-from sympy import oo
+from pystencils.backends.cbackend import CBackend
+
+
+class UnsupportedNode(pystencils.astnodes.Node):
+
+    def __init__(self):
+        super().__init__()
 
 
 @pytest.mark.parametrize('type', ('float32', 'float64', 'int64'))
@@ -12,9 +19,9 @@ def test_print_infinity(type, negative, target):
     x = pystencils.fields(f'x:  {type}[1d]')
 
     if negative:
-        assignment = pystencils.Assignment(x.center, -oo)
+        assignment = pystencils.Assignment(x.center, -sp.oo)
     else:
-        assignment = pystencils.Assignment(x.center, oo)
+        assignment = pystencils.Assignment(x.center, sp.oo)
     ast = pystencils.create_kernel(assignment, data_type=type, target=target)
 
     if target == pystencils.Target.GPU:
@@ -23,3 +30,23 @@ def test_print_infinity(type, negative, target):
     ast.compile()
 
     print(ast.compile().code)
+
+
+def test_print_unsupported_node():
+    with pytest.raises(NotImplementedError, match='CBackend does not support node of type UnsupportedNode'):
+        CBackend()(UnsupportedNode())
+
+
+def test_print_subtraction():
+    a, b = sp.symbols("a b")
+
+    x = pystencils.fields(f'x: double[3d]')
+    y = pystencils.fields(f'y: double[3d]')
+
+    config = pystencils.CreateKernelConfig(target=pystencils.Target.CPU)
+    update = pystencils.Assignment(x.center, y.center - b)
+
+    ast = pystencils.create_kernel(update, config=config)
+
+    code = pystencils.get_code_str(ast)
+    assert "-1.0" not in code