I am trying to prevent my users from skipping stages.  I have created this rule but I keep getting errors.  Can someone help me?
OR(and(
    ispickval<>(priorvalue(stage), "Presented Program")
   ,ispickval(stage,"Statement Collected")
),and(
    <>ispickval(priorvalue(stage), "Statement Collected")
   ,ispickval(stage,"Statement Analyzed")
),and(
    <>ispickval(priorvalue(stage), "Statement Analyzed")
   ,ispickval(stage,"Analyzer Presented")
),and(
    <>ispickval(priorvalue(stage), "Analyzer Presented")
   ,ispickval(stage,"Closed Won")
))

I am trying to create a field that sets a date to the 1st of the next month after an opportunity is marked close won.  I am not sure how to create that formula.