Hi!

I have the following

```
#!/usr/bin/env python3
from brian2 import *
@check_units(x=1, result=1)
def heaviside(x):
return (x > 0).astype(int)
neuron = NeuronGroup(1, "dv/dt = -v/(1*ms) : 1", method='euler')
ssg = SpikeGeneratorGroup(1, [0], [0*ms])
syn = Synapses(ssg, neuron, "dx/dt = 3/ms*heaviside(x - 1) - 4/ms*heaviside(-(x+2)) : 1")
run(1*ms)
```

Which fails with

```
NotImplementedError: The expression "3/ms*heaviside(x - 1) - 4/ms*heaviside(-(x+2))" contains more than one call of heaviside, this is currently not supported since heaviside is a stateful function and its multiple calls might be treated incorrectly (e.g."rand() - rand()" could be simplified to "0.0").
```

How do I tell Brian2 that my function is not stateful?

Thanks,

Sebastian