code: function P(i,l,a,b,R_1,R_lm1) {
      
      var ri1, rim1, ri0, ret;
      ri1 = R_1[i+1][1+1];
      rim1 = R_1[i+1][-1+1];
      ri0 = R_1[i+1][0+1];
      
      if (b==-l) {
          ret = ri1*R_lm1[a+l-1][0] + rim1*R_lm1[a+l-1][2*l-2];
      }
      else {
          if (b==l) ret = ri1*R_lm1[a+l-1][2*l-2] - rim1*R_lm1[a+l-1][0];
          else ret = ri0*R_lm1[a+l-1][b+l-1];
      }
      return ret;
  }
docs: function to compute term P of U,V,W (Table II)