@+(value, value) ; listable
@+ is the prefix form of the infix binary operator +
@+(x, y) ≡ x + y
The functional form of the operator is useful as an argument of a
high-order function as in @reduce(@+, v) which sums up all the
elements of the tab v.
The addition of an int and a float returns a float.
The addition of two string corresponds to the concatenation of the arguments.
The addition of a string and any other value convert this value into its
string representation before the concatenation.