private static Boolean canBeTrue(
    BooleanExpression b, 
    Map<String, Boolean> m) {
    // base cases... (e evaluates to true or false (not null))
    if (...) {
    }
    // recursive step...
    else {
    // for all possible choices...
        // for all possible truth values...
            // try that assignment (e.g. a=true)
            // recursively try the rest
            // undo that assignment if we failed to get an answer
    }
}
Solving a recursive backtracking problem:
    (1) What are the choices we need to make?
        choosing T or F for each variable 
    (2) How do we make a choice?
        take v, assign x--> m.put(v, x);
    (3) How do we undo a choice?
        m.remove
    (4) When do we stop / what are our base cases?
        (4a) Success base case(s)?
            b evaluates to true
        (4b) Failure base case(s)?
            b evaluates to false
Good examples:
    (a && !a), {}      -> false
    (a||!a), {a=true}  -> true 
    (a||!b), {a=false} -> null