Nullpointer exception on theory append

[Philipp Kraus]

This codeline in the Theory class creates an null pointer exception

this.theory = this.theory.toString() + "\n" + th;

If I create a theory with a struct and append a theory with a string

Theory s = new Theory( new Struct(<some data>) );
Theory t = new Theory( "doit(X) :- X, X > 5." );

s.append( t ) 

the call

this.theory.toString()

create the NPE, because the theory member is set to null. the problem can be solved if the string member theory is set to an empty string on the top of the class. I recommand that the clauseList is also set to an empty object, to avoid NPEs