-- a tiny context-free grammar for English
-- subcategories with _ in them are an unscalable hack

-- combination rules

S ::= NP_subj_3 VP_3 | NP_subj VP | S Adv ;

VP ::= V2 NP_obj ;
VP_3 ::= V2_3 NP_obj ;

NP_subj_3 ::= Det CN | Pron_subj_3 ;
NP_subj ::= Pron_subj ;
NP_obj ::= Det CN | Pron_obj ;

CN ::= A N ;

Adv ::= Subj S ;

-- structural words

Det ::= "the" ;

Pron_subj ::= "we" ;
Pron_obj ::= "us" ;

Subj ::= "because" ;

-- content words

N ::= "cat" ;
A ::= "black" ;
V2_3 ::= "sees" ;
V2 ::= "see" ;
Adv ::= "now" ;