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    $\begingroup$ First@Cos[a]? $\endgroup$ Commented Jun 9, 2014 at 0:03
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    $\begingroup$ Note that all expressions have heads and bodies. These two are not separable. For example, a has head Symbol. All you can do is to replace a head with something else or extract parts of an expression. $\endgroup$ Commented Jun 9, 2014 at 0:50
  • $\begingroup$ @Oleksandr I agree and would think that the closest thing to a headless expression is one with Sequence as head. $\endgroup$ Commented Jun 9, 2014 at 2:36
  • $\begingroup$ @acl Identity $\endgroup$ Commented Jun 9, 2014 at 4:18
  • $\begingroup$ @MikeHoneychurch I was thinking about things like Sequence @@@ List[List[a, b], List[a, c]]. But you're right, for examples like in your answer Identity does the job. $\endgroup$ Commented Jun 9, 2014 at 12:04