式木の定義的なアレ

expr_tree.als

abstract sig Expr {
    lhs : lone Expr,
    rhs : lone Expr
}
sig True  extends Expr {}
sig False extends Expr {}
sig Not extends Expr {}
sig And extends Expr {}
sig Or  extends Expr {}

fun children [e : Expr] : Expr {
    e.^(lhs + rhs)
}

fun family [e : Expr] : Expr {
    e.*(lhs + rhs)
}

pred op_prim[e : Expr] {
    no children[e]
    e in (True + False)
}

pred op_not[e : Expr] {
    one e.lhs and no e.rhs
    e in Not
}

pred op_two[e : Expr] {
    one e.lhs and one e.rhs
    e in (And + Or)
}

fact tree {
    one root : Expr | Expr = family[root]
    all e : Expr {
        no family[e.lhs] & family[e.rhs]
        not e in children[e]
    }
}

fact operator {
    all e : Expr | op_prim[e] or op_not[e] or op_two[e]
}

pred show {
}

run show for 8 Expr