Corrected eroneous proof. Created VariableFormulaLabel to replace arity-0 predicate schemas.

Any work on top of current LISA release should be easily adaptable by:
 - Changing all instances of Arity 0 SchematicFunctionLabel by VariableLabel
 - Changing all instances of Arity 0 SchematicPredicateLabel by VariableFormulaLabel
 - Changing all matching on SchematicFunctionLabel by either VariableLabel, SchematicFunctionLabel or SchematicTermLabel, according to whether the arity should be =0, >0 or >=0.
 - Changing all instances of multiary SchematicPredicateLabel by SchematicNPredicateLabel
 - Removing the empty parenthesis () that where used to transform an Arity 0 label to a term or formula. Variables have the implicit conversion without the call.