Editing Vienna Development Method (section)

===Natural number multiplication===
<syntaxhighlight lang="rsl">
 multp(i,j:nat)r:nat
 pre true
 post r = i*j
</syntaxhighlight>
Applying the proof obligation <code>forall p:T_p & pre-f(p) => f(p):T_r and post-f(p, f(p))</code> to an explicit definition of <code>multp</code>:
<syntaxhighlight lang="rsl">
 multp(i,j) ==
  if i=0
  then 0
  else if is-even(i)
       then 2*multp(i/2,j)
       else j+multp(i-1,j)
</syntaxhighlight>
Then the proof obligation becomes:
<syntaxhighlight lang="rsl">
 forall i, j : nat & multp(i,j):nat and multp(i, j) = i*j
</syntaxhighlight>
This can be shown correct by:
# Proving that the recursion ends (this in turn requires proving that the numbers become smaller at each step)
# [[Mathematical induction]]