Implicit and explicit input bindings

Are the following pipelines logically the same? Are they all legal?

  <p:pipeline name="explicit">
    <p:input port="source" primary="yes"/>
    <p:output port="result" primary="yes"/>

    <p:identity>
      <p:input port="source">
        <p:port step="explicit" port="source"/>
      </p:input>
    </p:identity>
  </p:input>

  <p:pipeline name="semi-implicit">
    <p:input port="source" primary="yes"/>
    <p:output port="result" primary="yes"/>

    <p:identity>
      <p:input port="source"/>
    </p:identity>
  </p:input>

  <p:pipeline name="fully-implicit">
    <p:input port="source" primary="yes"/>
    <p:output port="result" primary="yes"/>

    <p:identity/>
  </p:input>

Several times now, I've gotten the impression that some people feel
that

  <px:step-name>
    <p:input name="foo"/>
  </px:step>

is semantically different from

  <px:step-name/>

and I don't think it should be. If you think it is, please explain how
and why.

                                        Be seeing you,
                                          norm

-- 
Norman Walsh <ndw@nwalsh.com> | Before doing someone a favour, make
http://nwalsh.com/            | sure that he isn't a madman.--Eugéne
                              | Labiche