Project Ocaml (help)

/*I want to implement in Ocaml a Boolean solver, that is to say an algorithm to calculate the set of solutions of a system of boolean equations. the first step is to determine the set of variables in the system of equations. The first will result in the list [V1; V2; V3] . I code this but it's false , i don't know how to begin */

# type eb= Vof int | TRUE | FALSE| AND of eb * eb | OR of eb * eb | XORof eb* eb | NOTof eb;;
let rec question1 eb =
let l=[] in
match eb with
|OR(a,b)-> if (OR(a,b)=TRUE) then ((l=[(a,TRUE);(b,TRUE)]) || (l=[(a,TRUE);(b,FALSE)]) || (l=[(a,FALSE);(b,TRUE)]))
            else (l=[(a,FALSE);(b,FALSE)])
|AND(a,b)-> if (AND(a,b)=TRUE) then (l=[(a,TRUE);(b,TRUE)])
            else ((l=[(a,FALSE);(b,FALSE)]) || (l=[(a,TRUE);(b,FALSE)]) || (l=[(a,FALSE);(b,TRUE)]))
|NOT(a)-> if (NOT(a)=TRUE)  then (l=[(a,FALSE)])
            else (l=[(a,TRUE)])
|XOR(a,b)-> if (XOR(a,b)=TRUE) then ((l=[(a,TRUE);(b,FALSE)]) || (l=[(a,FALSE);(b,TRUE)]))
                else ((l=[(a,FALSE);(b,FALSE)]) || (l=[(a,TRUE);(b,TRUE)]))
;;