A block:

    if a
        do_stuff
    end

A prefix:

    given x = a+b
    do_stuff x, x

An infix expression:

    do_stuff (b1 xor b2)

A prefix/function-like expression:

    do_stuff (default x, -1)


Should all keywords be "locked" to be either:
- a block
- a prefix
- an infix expression
- a prefix expression

Some proglangs use a sigill or some trailing suffix (e.g. `:` or `|`...`|`)

The `given` expression is already special, with the `=` in it.