AttributeError: 'Dummy' object has no attribute 'type_'

This issue has occurred after solving issue https://git.scc.kit.edu/virtmat-tools/vre-language/-/issues/53.

This example (obviously not in the tests)

sqr(x) = x*x
z_minus_sqr(q, r) = 2*sqr(-1*q+r)
print(z_minus_sqr(2, 4.))

raises AttributeError: 'Dummy' object has no attribute 'type_'. But it works in this ordering

print(z_minus_sqr(2, 4.))
z_minus_sqr(q, r) = 2*sqr(-1*q+r)

This is due to the missing substitution done by children list of metamodel type VarReference. After the fist subsitution (the first function call) ref points to objects other than Variable or Dummy. This is why these objects are not catched in the second substitution (in the second function call).

There should be differentiation in the if statement between these cases.