Question on Skill Code for Cadence Official MOS PCell

Hi,

Cadence provided us with the Skill code for the MOS PCell. I've been learning it and ran into some issues, so I'd appreciate your help. Thanks.

Known information (from Cadence official):

;;substrateContact? specifies whether contacts are necessary or not on the
;; source and drain of the transistor, and then creates it.

;;leftContact? the left side of the transistor be it the source or
;; drain

;;rightContact? the right side of the transistor be it the source or
;; drain

;;transistor_length length of the transistor's active area
;; (note: this is a distance measured in the X-direction).
;; (float, default = 1.0)

;; car(substrateRules) = tap diffusion layer.
;; cadr(substrateRules) = tap diffusion contact enclosure.
;; caddr(substrateRules) = tap diffusion to active spacing.
;; cadddr(substrateRules) = tap diffusion to gate poly spacing.
;; car(cddddr(substrateRules)) = tap implant rules.

;;contactWidth the width of the contact layer being used for
;; connections
;; (default = 1.0)

(leftStretch techGetSpacingRule(techGetCurrentTechFile() "minSpacing"
     techParam("poly") techParam("cont")) + techGetSpacingRule(
     techGetCurrentTechFile() "minWidth" techParam("cont")) +
     techGetOrderedSpacingRule(techGetCurrentTechFile() "minEnclosure"
     techParam("ndiff") techParam("cont")) )

(rightStretch techGetSpacingRule(techGetCurrentTechFile() "minSpacing"
     techParam("poly") techParam("cont")) + techGetSpacingRule(
     techGetCurrentTechFile() "minWidth" techParam("cont")) +
     techGetOrderedSpacingRule(techGetCurrentTechFile() "minEnclosure"
     techParam("ndiff") techParam("cont")) )

Code segment with questions (from Cadence official):

;;*************************************************************************
;;
;;The following adjusts the rightStretch to account for the substrateContact
;;---------------------------------------------------------------------
;;
;;*************************************************************************
when( substrateContact? && substrateContact? !="FALSE"
         if( leftContact? && leftContact? != "FALSE" &&
             rightContact? && rightContact? != "FALSE" then
         else
             xtrWidth = leftStretch + transistor_length + rightStretch
             minSubTie = contactWidth + 2 * cadr(substrateRules)
           ; fprintf( stdout "xtrWidth minSubTie %L\n" list(xtrWidth minSubTie) )
             when( xtrWidth < minSubTie
                      rightStretch = rightStretch + minSubTie - xtrWidth
                      )
         )
)

My Questions:

1.The source code uses an if-then-else structure. I believe the correct form should be either if-then or if-else.

2.What is the purpose of this piece of code? Why does it only adjust rightStretch, with no case to adjust leftStretch?

3.I am particularly confused by this line of code:

             when( xtrWidth < minSubTie
                      rightStretch = rightStretch + minSubTie - xtrWidth
                      )

Why do we need to consider the scenario where xtrWidth is smaller than minSubTie? And why is rightStretch calculated as rightStretch + minSubTie - xtrWidth under this condition?

I would appreciate your insights. Thank you.
Steve