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Predicate eval_default/4
 eval_default(+Default, +Module, +Type, -Value) is det[private]
Convert the settings default value. The notation allows for some `function-style' notations to make the library more generic:
Get value from the given environment variable. The value is handed to convert_setting_text/3 to convert the textual representation into a Prolog term. Raises an existence_error of the variable is not defined.
env(Name, Default)
As env(Name), but uses the value Default if the variable is not defined.
Ask the value of another setting.
If Type is numeric, evaluate the expression. env(Var) evaluates to the value of an environment variable. If Type is atom, concatenate A+B+.... Elements of the expression can be env(Name).