param D=1
param dx=1
c0'=D*(c1-c0)/(dx*dx)
c[1..39]'=D*(c[j+1]-2*c[j]+c[j-1])/(dx*dx)
c40'=D*(c39-c40)/(dx*dx)
init c[0..19]=0
init c20=0.5
init c[21..29]=1
init c30=0.5
init c[31..40]=0