v1.6.2.3 fix: error in lag formulation

- Following correction proposed #34 (comment 22395)
- Correct tests consequently

Refs #34
3 jobs for sd in 10 minutes and 44 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #104669
build_patched

00:00:20

 
  Tests
passed #104671
check_as_cran_patched

00:01:27

passed #104670
check_not_cran_patched

00:08:55