unmodified = ['big oh', 'order', 'order of complexity', 'time']
O = bnf_expand([asymtotic, asymtotic_modified],
[common_modifiers, commonly_modified], [unmodified])
unmodified = ['big oh', 'order', 'order of complexity', 'time']
O = bnf_expand([asymtotic, asymtotic_modified],
[common_modifiers, commonly_modified], [unmodified])