Final fix for the linear state updater, should now work with sympy 0.…

…7.2 and 0.7.3

brian2/stateupdaters/exact.py

@@ -237,8 +237,8 @@ def __call__(self, equations, specifiers=None):
         dt = Symbol('dt', real=True, positive=True)
         A = (matrix * dt).exp()                
         C = sp.ImmutableMatrix([A.dot(b)]) - b
-        S = sp.MatrixSymbol('_S', len(variables), 1)
-        updates = A * S + C.transpose()
+        _S = sp.MatrixSymbol('_S', len(variables), 1)
+        updates = A * _S + C.transpose()
         try:
             # In sympy 0.7.3, we have to explicitly convert it to a single matrix
             # In sympy 0.7.2, it is already a matrix (which doesn't have an
@@ -247,11 +247,11 @@ def __call__(self, equations, specifiers=None):
         except AttributeError:
             pass
 
-        # The solution contains _S_00, _S_10 etc. for the state variables,
+        # The solution contains _S[0, 0], _S[1, 0] etc. for the state variables,
         # replace them with the state variable names 
         abstract_code = []
         for idx, (variable, update) in enumerate(zip(variables, updates)):
-            rhs = update.subs('_S_%d0' % idx, variable)
+            rhs = update.subs(_S[idx, 0], variable)
             identifiers = get_identifiers(sympy_to_str(rhs))
             for identifier in identifiers:
                 if identifier in specifiers: