choosing pool based on cookie exists+cookie value

can you please help me correct the errors in my last part of the script:

when HTTP_REQUEST {

if {[HTTP::cookie exists "cat"]}{

  if {[HTTP::cookie value "cat"]equals "yes"}{

    pool Pool_ID

} }

here is the errors:

Rule [iRule-corder] error: line 15: [wrong args] [when HTTP_REQUEST{] line 17: [command is not valid in the current scope] [if {[HTTP::cookie exists "F2C"]}{ if {[HTTP::cookie value "F2C"]equals "yes"}{ pool Pool_VIP2 } }]