flow control

   The help page for ?"for" says that:

   The index seq in a for loop is evaluated at the start of the loop;
changing
   it subsequently does not affect the loop. The variable var has the
same type
   as seq, and is read-only: assigning to it does not alter seq.

The help file is not right when seq is a list() or other recursive
type.  In that case var has the type of seq[[i]] where i is the current
iteration count.  (I think this is true in general, since [ and [[ act
the same for nonrecursive types when the indices are such that a scalar
would be returned.  However that explanation is unnecessarily
complicated
in the nonrecursive case.)

Also, the variable var is not really read-only.  You can alter it but
it gets reset to the next value in seq at the start of each iteration.
You cannot affect the meaning of 'next' to force it to, e.g, omit or
repeat iterations.