feature request: formulae

I understand the desire to avoid functions, but they do allow code reuse. A happy medium would be formulae, that is names attached to a specific expression. It could be as simple as

```
> sqr := a * a
formula sqr created
> a = 5
ans = 5.0
> sqr
ans = 25.0
```

or, with a little more work, as nice as

```
> sqr(a) := a * a
formula sqr created
> sqr(5)
ans = 25.0
```