Retain statement-- previous row value based on two other variables

[EpiNovice]

Data sample: 

ID	Test	VisitType
1	0	S
1	1	S
1	1	U
1	0	S
1	1	S
1	1	S
1	1	S
2	1	S
2	1	S
2	0	S
2	1	S
2	1	U
2	1	U
2	1	U
2	1	U
2	0	S
2	0	S
2	0	S

Based on the above data, I'm trying to create a variable (VarWant) that, for each ID, is a 1 if VisitType=S and Test=1 OR is a 1 if VisitType=S, Test=0, BUT the preceding VisitType=U and Test=1. For clarification, here is what I'm looking for:

ID	Test	VisitType	VarWant
1	0	S	0
1	1	S	1
1	1	U	1
1	0	S	1
1	1	S	1
1	1	S	1
1	1	S	1
2	1	S	1
2	1	S	1
2	0	S	0
2	1	S	1
2	1	U	1
2	1	U	1
2	1	U	1
2	1	U	1
2	0	S	1
2	0	S	0
2	0	S	0

I know I can use a retain statement for this, but I can't figure out the logic statement that should accompany it. Thanks in advance for the help!

[Astounding · Accepted solution]

The LAG function should make this straightforward:

data want;
   set have;
   by id;
   prior_type = lag(VisitType);
   prior_test = lag(test);
   if first.id then call missing(prior_type, prior_test);
   VarWant = (VisitType="S" and test=1) or (VisitType="S" and test=0 and prior_type="U" and prior_test=1);
run;

[EpiNovice]

This worked beautifully, thank you!!