# Substituting function value in an expression

I have an expression like `uR(t) == 3*iL(0) + uC(0)/2 - 4`

how can I substitute for `iL(0)`

and `uC(0)`

, if I'm given, that `iL(0) = 0`

and `uC(0) = 0`

.

`uR`

, `iL`

, `uC`

are a functions of var `t`

:

```
t = var('t')
uR = function('uR', t)
iL = function('iL', t)
uC = function('uC', t)
```

thank you :)