expression(t)
Return the next compiled Slogan expression from the stream tokenizer t.
If no more expressions can be read from the tokenizer, return the eof object.
let s = string_reader(read_line())
// enter some expression, say for example, `1 + 2`
let t = stream_tokenizer(false, s)
let e = expression(t)
e
// a system dependent compiled representation of the expression.
eval(e)
// 3