* accessor.test: Added tests to document the behaviour for internally generated per-slot accessor methods * nx.tcl (VariableSlot), properties.test: Make sure that calling delete on a single-valued property/ variable errors out, like calling add * nx.tcl (defineIncrementalOperations): Rewrite to avoid repetitive computations * nx.tcl: First take on fixing definition of incremental slot operations, in presence of custom provided ones (esp. value=delete)