question: public double theta ( final double spot , final double maturity ) { QL . require ( maturity > 0.0 , "non negative maturity required" ) ; if ( maturity == 0.0 ) return 0.0 ; return - ( Math . log ( discount ) * value ( ) + Math . log ( forward / spot ) * spot * delta ( spot ) + 0.5 * variance * spot * spot * gamma ( spot ) ) / maturity ; }
target:  Sensitivity to time to maturity.