   #  
   #  chr=0 for x1<=1 and chr=1 x1>0
   #
      chr=max(0,1.-x1)/max(1.d-10,1.-x1)
   #
   #     / 900.          x1<=1.
   #  k=|         for           = thermal conductivity
   #     \\   1.          x1>1.
   #
      k= chr * 900. + (1-chr) * 1.
   #
   #  no heat generation:
   #
      qb=0
   #
   #  convection boundary condition:
   #
       qs= (u1 - 20) ^ 2 
   #
   #  the prescribed temperature is given by the input file:
   #
      u1=prevalue
   #
   #  and here is variational problem:
   #
     volume { k/2 * ( u1x1^2 + u1x2^2 ) - qb*u1 } +
     area { qs/2 } = minimal
