Modified the netgen setup for sky130 per Mitch Bailey's observation
that the 16V and 20V devices are extended-drain devices and so
should not be allowed to permute source and drain terminals.

sky130/netgen/sky130_setup.tcl

diff --git a/sky130/netgen/sky130_setup.tcl b/sky130/netgen/sky130_setup.tcl
index 817ff0a..2162a39 100644
--- a/sky130/netgen/sky130_setup.tcl
+++ b/sky130/netgen/sky130_setup.tcl
@@ -140,9 +140,6 @@
 lappend devices sky130_fd_pr__cap_var_lvt
 lappend devices sky130_fd_pr__cap_var_hvt
 lappend devices sky130_fd_pr__cap_var
-lappend devices sky130_fd_pr__nfet_20v0_nvt
-lappend devices sky130_fd_pr__nfet_20v0
-lappend devices sky130_fd_pr__pfet_20v0
 
 foreach dev $devices {
     if {[lsearch $cells1 $dev] >= 0} {
@@ -166,6 +163,39 @@
 }
 
 #---------------------------------------------------------------------
+# Extended drain MOSFET devices.  These have asymmetric source and
+# drain, and so the source and drain are not permutable.
+#---------------------------------------------------------------------
+
+set devices {}
+lappend devices sky130_fd_pr__nfet_20v0_zvt
+lappend devices sky130_fd_pr__nfet_20v0_nvt
+lappend devices sky130_fd_pr__nfet_20v0_iso
+lappend devices sky130_fd_pr__nfet_20v0
+lappend devices sky130_fd_pr__pfet_20v0
+lappend devices sky130_fd_pr__nfet_g5v0d16v0
+lappend devices sky130_fd_pr__pfet_g5v0d16v0
+
+foreach dev $devices {
+    if {[lsearch $cells1 $dev] >= 0} {
+	property "-circuit1 $dev" parallel enable
+	property "-circuit1 $dev" parallel {l critical}
+	property "-circuit1 $dev" parallel {w add}
+	property "-circuit1 $dev" tolerance {w 0.07} {l 0.01}
+	# Ignore these properties
+	property "-circuit1 $dev" delete as ad ps pd mult sa sb sd nf nrd nrs area perim topography
+    }
+    if {[lsearch $cells2 $dev] >= 0} {
+	property "-circuit2 $dev" parallel enable
+	property "-circuit2 $dev" parallel {l critical}
+	property "-circuit2 $dev" parallel {w add}
+	property "-circuit2 $dev" tolerance {w 0.07} {l 0.01}
+	# Ignore these properties
+	property "-circuit2 $dev" delete as ad ps pd mult sa sb sd nf nrd nrs area perim topography
+    }
+}
+
+#---------------------------------------------------------------------
 # (MOS) ESD transistors.  Note that the ESD transistors have a flanged
 # gate.  Magic disagrees slightly on how to interpret the width of the
 # devices, so the tolerance is increased to 7% to cover the difference