True if Value is an atom associated with the environment variable
or system property Name.
Source may be properties (to get only system properties),
environment (to get only environment variables),
or merged (the default, to get both, with system properties
taking precedence over environment variables of the same name).
- Compatibility
- - SICStus 4
- To be done
- - Mode -Name is not supported.
Because SWI-Prolog doesn't have an obvious equivalent to
SICStus system properties, these predicates currently
behave as if no system properties are defined,
i. e. only environment variables are returned.
