Brian implements the short-term plasticity model described in: Markram et al (1998). Differential signaling via the same axon of neocortical pyramidal neurons, PNAS 95(9):5323-8. Synaptic dynamics is described by two variables x and u, which follows the following differential equations:

```
dx/dt=(1-x)/taud (depression)
du/dt=(U-u)/tauf (facilitation)
```

where taud, tauf are time constants and U is a parameter in 0..1. Each a presynaptic spike triggers modifications of the variables:

```
x->x*(1-u)
u->u+U*(1-u)
```

Note that the update order is important. Synaptic weights are modulated by the product u*x (in 0..1), which is taken before updating the variables. This model describes both depression and facilitation.

To introduce short-term plasticity into an existing connection C, use the class
`STP`:

```
mystp=STP(C,taud=100*ms,tauf=5*ms,U=.6)
```