effect of p:compare

p:compare is one of those step which has no primary output port

In 2.5 Environment,

[[
If there is a preceding sibling step element:

   -

   If that preceding sibling has a *primary output
port<http://www.w3.org/XML/XProc/docs/langspec.html#dt-primary-output-port>
   *, then that output port becomes the *default readable
port<http://www.w3.org/XML/XProc/docs/langspec.html#dt-default-readable-port>
   *.
   -

   Otherwise, the *default readable
port<http://www.w3.org/XML/XProc/docs/langspec.html#dt-default-readable-port>
   * is undefined.

]]

does it mean that

<p:identity/>
<p:compare/>
<p:identity/>

1) will throw an error on the second p:identity, because the default
readable port is undefined
2) will use the last defined "default readable port", which is the result of
the first identity

Mohamed
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