#1

solvable(1) => 1;                         // 1 is solvable
solvable(["+" | L]) => some(solvable, L); // + rooted solvable if some subtree is
solvable(["*" | L]) => all(solvable, L);  // * rooted solvable if each subtree is
solvable(_) => 0;                         // nothing else is solvable