 optional string to be concatenated to the beginning
of any received data before return.
</p>

<p class=return>
If successful, the method returns the received pattern. In case of error,
the method returns <tt><b>nil</b></tt> followed by an error
message, followed by a (possibly empty) string containing
the partial that was received. The error message can be
the string '<tt>closed</tt>'  in   case  the  connection  was
closed  before  the transmission  was completed  or  the string
'<tt>timeout</tt>' in  cas