Return a pseudo-random Number generated with an xoshiro128** algorithm using seed {expr}. The returned number is 32 bits, also on 64 bits systems, for consistency. {expr} can be initialized by |srand()| and will be updated by rand(). If {expr} is omitted, an internal seed value is used and updated. Returns -1 if {expr} is invalid. Examples: :echo rand() :let seed = srand() :echo rand(seed) :echo rand(seed) % 16 " random number 0 - 15