Data often needs to be grouped together, for example, consider a list of
icons. Sometimes a trick like arranging little images into a big raster
works, but generally they'll need to be structured as a first class group.
The objects "list" and "cat " are iff-universal mechanisms for this
purpose.  Note: list and cat are advanced topics the first time reader
will want to skip.

Property settings sometimes need to be shared over a list of similar
objects. E.g., a list of icons may share one color map.  list provides a
means called "prop" to do this.  one purpose of a list is to define the
scope of a PROP. A "cat ", on the other hand, is simply a concatenation of
objects.

Simpler programs may skip lists and props altogether and just handle forms
and cat s.  all "fully-conforming" iff programs also know about "cat ",
"LIST", and "PROP".  Any program that reads a form inside a list must
process shared PROPs to correctly interpret that FORM.

 group cat     group list     group prop     properties for list