Leaf nodes of propositional feature expressions, either an external feature defined by the user or another feature expression from a macro.
external definition of a feature (cannot be decided to Base or Dead inside this file)
definition based on a macro, still to be resolved using the macro table (the macro table may not contain DefinedMacro expressions, but only DefinedExternal) assumption: expandedName is unique and may be used for comparison
Propositional (or boolean) feature expressions.
the feature model is a special container for a single feature expression that is used very often in a conjunction
connection to the SAT4j SAT solver
CNFHelper provides several auxiliary functions to determine whether an expression is in normal form and to access parts of that normal form
companion object to create feature models
empty feature model
True and False.