s an abstract container derived type which can be used 
as a sort of unlimited polymorphic entity whose contents are
retrievable with type-guards. Different subclasses are created
in order to hold different data-types. Subclasses are provided for
the default variable types in Fortran. The abstract type can be used
within data structures to so that a single implementation can hold
arbitrary types of contents. The type of the contents can be selected
when the data structure is constructed, by passing a partic