int  expandvertexgroup(<geometry>geometry, string groupname)
int  expandvertexgroup(<geometry>geometry, string groupname, string mode)
This can use ad-hoc groups, like
This uses the SOP group naming convention, in particular that an empty string means all.
mode can be
ordered is the default mode and will return numbers in the order of appearance in the string, but only for numbers. The order won’t be kept when using expressions such as
@Cd.x>0.5. The same number won’t appear twice in returned array.
unordered mode returns the resolved group following sorted point numbers order.
split mode starts by splitting the
groupname string on
@ characters and then does one resolution per sub string. The order is kept between the sub strings, but will fallback to unordered when resolving a group expression. This same number can appear multiple time when resolving using this mode.