assert(simp(ZERO | ONE) == ONE)
assert(simp(STAR(ZERO | ONE)) == STAR(ZERO | ONE))
assert(simp(ONE ~ (ONE ~ (ONE ~ CHAR('a')))) == CHAR('a'))
assert(simp(ONE ~ (ONE ~ (ONE ~ ZERO))) == ZERO)
assert(simp(ALT(ONE ~ (ONE ~ (ONE ~ ZERO)), CHAR('a'))) == CHAR('a'))
assert(simp(CHAR('a') | CHAR('a')) == CHAR('a'))
assert(simp(ONE | CHAR('a')) == (ONE | CHAR('a')))
assert(simp(ALT((CHAR('a') | ZERO) ~ ONE,
               ((ONE | CHAR('b')) | CHAR('c')) ~ (CHAR('d') ~ ZERO))) == CHAR('a'))