Incrementing a list by 2 (or more)

[Grant Ingersoll]

I have a flattened list (guaranteed to always be even in length) of points and I'm trying to generate WKT (Well Known Text) from it which looks like.  So, for a list like:

0 = {java.lang.Double@23771} "21.0"

1 = {java.lang.Double@23772} "41.0"

2 = {java.lang.Double@23773} "22.0"

3 = {java.lang.Double@23774} "42.0"

4 = {java.lang.Double@23775} "23.0"

5 = {java.lang.Double@23776} "43.0"

6 = {java.lang.Double@23777} "24.0"

7 = {java.lang.Double@23778} "43.0"

8 = {java.lang.Double@23779} "21.0"

9 = {java.lang.Double@23780} "41.0"

I need my string to look like

POLYGON ((21.0 41.0, 22.0 42.0, 23.0 43.0, 24.0 44.0, 21.0 41.0))

In order to do so, I need to increment by two at a time through my list, outputting the ith and (i+1)th elements as pairs, separated by a comma. 

If conditionals could evaluate for more than just presence of an attribute, something like:

POLYGON ((<geometry_coordinates:{n|<i0>: <if(i0)><n> <elseif(!i0)><n>, <endif>}>)) //notice the "not" operator

but this doesn't work, per the docs at https://github.com/antlr/stringtemplate4/blob/master/doc/templates.md#conditionals

As added restriction, I'm using software that has ST4 embedded in it, so I don't have any control over the underlying Java code, so I have to be able to do it just through the ST4 syntax.

Thanks,

Grant