AM_MOD_SYMLINE modifies existing symmetry lines by extending or shrinking them.
+----------------<-----------------+
| |
| |
---->(AM_MOD_SYMLINE)---+---|symmetry line|---|position|---+----->
This command has no options. Simply indicate a symmetry line and then a point
for it to extend or shrink to.
Use this action to extend or shrink existing symmetry lines.
AM_CREATE_SYMLINE terminate action