Functions to operate on property lists.
Trying to handle property lists with 'empty' keys:
(:a 1 :b 2 :c :d 4)
^ <-- this is the 'empty' key
So the keywords can't be the values of keys.
It's used in bptr-html and looks like working well.
But I think it's very buggy beyond functions that i used in bptr-html ;p.