Constraint for most recent four numbers are unique out of stream of random number

SystemVerilog
8

In reply to megamind:

Another question is from the slide title as “Distribution Example”

op dist {READ := 20 , WRITE := 15 , NOP := 5};
And you said READ = 50% , WRITE = 40% and NOP = 20%
How do you calculate % value from the distribution numbers ?
May be I am missing something fundamental, it will be a great help if you can quick explain or share the relevant information, so I can understand this topic completely.

Thank you.
Mega