A property list is a collection of equations (substitution rules) and transforms.
Like transforms and paragraphs, a property list is a top-level construct.
Its input form contains an introductory = operator followed by a comma-separated
list. The first element of the list may be a label used to identify the property list.
The remaining elements are either equations that conform to the requirements of substitution
rules or transforms.
The use of property lists is described in §3.3.6.
The management of the property list collection is described in §8.1.8.
