Passing multi-value param to subreports

Question

I have a main report that has 3 subreports. It used to take a parameter, which was in each subreport and main report sql commands, each would bring back records pertinent to that param.
Now report needs to be able to take a multi-value param. I took it out of sql commands and have record selection handle it in CR directly (in all subs and main). Problem is data is displayed like this
Given Params : p1, p2
for p1 //main (here grouping on field, which is the param)
rec1 rec2 // Sub a - displays all records for both param - bad, record selection doesnt do what i want
rec1 rec2 // Sub b - same
for p2 // main
rec1 rec2 // Sub a - same
rec1 rec2 // Sub b - same
what I need is
for p1 //main
rec1 // Sub a - want to do record selection only to rows associated with p1
rec1 // Sub b
for p2 // main
rec2 // Sub a
rec2 // Sub b
I am linking main report to subreports on that param field, but subs display the whole nine yards, ie for all values in the multi-value list of params instead of just "current" param. What to do?

Answer 1

Thank you for prompt response.  Would you know the specificis of how to accomplish this?  I've removed the param link between main and sub. 
Then in main I created a formula "put" where I declared
Shared StringVar i := {Command.field}; --tryng to pass this to subreport
Then in subreport I created a formula "get" with
Shared StringVar i;
i;
However when I look at sub's record selection, trying to restrict selection like this
{SubCommand.field} = {@get}
my formula "get" is not available.  It is available for Group Selection, I tried to put above line in there, got error "This formula cannot be used because it must be evaluated later".  I'm definately not understanding how to pass one of multi-value params to sub.  Please help!