question: public double eval ( int id1 , int id2 , Instance inst1 ) throws Exception { double div = Math . sqrt ( super . eval ( id1 , id1 , inst1 ) * ( ( m_keys != null ) ? super . eval ( id2 , id2 , m_data . instance ( id2 ) ) : super . eval ( - 1 , - 1 , m_data . instance ( id2 ) ) ) ) ; if ( div != 0 ) { return super . eval ( id1 , id2 , inst1 ) / div ; } else { return 0 ; } }
target:  Computes the result of the kernel function for two instances. If id1 == -1, eval use inst1 instead of an instance in the dataset. Redefines the eval function of PolyKernel.