bool

while angle has keywords true and false, type bool is irrelevant since these are transparently mapped to numbers:
false == 0 == {} == () == ø // as in many mathematical axiom systems these are (near) identical**
see Truthiness

In case you are not familiar with axiomatic mathematics, here is why the identification is free of contradiction:

() + 1 == 0 + 1 == 1
() + 1 == (1) == 1  // as list concatenation