specify_models is a function that build up a specification of a
longitudinal data structure and associated models from simple data description
functions, such as
identify_outcome. The object returned by this function describes the the
key attributes of a longitudinal data structure, such as the names of the
treatment, censoring, and outcome variables. These are used by various
inferential functions. The function is meant to be general so that elements
can be added to it so that it can be used by future estimation functions.
This description adds meaning and describes structure of standardized
Descriptor functions passed as seperate arguments. See examples.
An object of type
model_specification that describes the data