what i should probably do is add 'hooks' just before newlines have been yielded
so you can look at the previous line and say hey it's an include
let's start lexing that instead

unify/optimize ascii/asciiz/byte/halfword/word into BIN directives
also lex strings to binary strings, why not

;pseudo-instr for offsets not within 0x8000 range?
addiu   at, t0, 0x7FFF
sb      t1, 0x62FE(at)
;versus
sb      t1, r0+0xE2FD(t0)

directive aliases, are these right?
    DB = 'BYTE',
    DH = 'HALFWORD',
    DW = 'WORD',

add basic command-line interface (patch.lua)

improve writer performance (just copypaste what you did in patch.lua)

procrastinating the heck over:
    write tests for everything (try to focus on code paths and edge cases)

low priority:
    improve parser terminology
    add delay slot warnings
    externally document more stuff like syntax