I don't think merlyn has a column on this, but he and tye had an exchange which marches smartly off to the right beginning here.

Here's a summing up from merlyn:
'If you keep "statement" and "expression" separate, it's obvious that "EXPR1 while EXPR2;" is a statement, not an expression, and cannot further be modified, and that "EXPR1 or EXPR2" is still an expression and can be used in a larger expression.'

  p