First Argument (element of a) whose implicit conversion to bool is true.
Similar to behaviour of or operator in dynamic languages such as Lisp's (or a...) and Python's a or ....
TODO Is inout Conversion!T the correct return value?
NOTE: Lazy parameters are currently marked as throw which makes it impossible to mark either() as nothrow. Issue at https://issues.dlang.org/show_bug.cgi?id=12647