t is empty, an undefined value is returned.
t has only one element, this element is returned.
In the other case, the binary operation is used to combine all the
t into a single value
f(... f(f(t, t), t), ... t[n])
For example, if
t is a vector of booleans,
@reduce(@||, t) returnsthe logical disjunction of thet`’s elements.