SAS code for running regressions within a loop-2

CRSP monthly returns are sorted by firm-month. Within each firm, we need to run a regression using the previous 60 observations (if available), and produce the residual of the regression as the observation variable of interest for the 61st observation. In other words, obs 1-60 are used to generate a res_value for the 61st observation. Similarly, obs 2-61 are used to generate a res_value for the 62nd observation, etc.