Passing empty parameters discard parameters default value

Hello,

I am having problems with passing empty parameters (no with-param specified) 
that discard default values of parameters. When I am calling the template and 
passing no value I want the template to use its default value.

Example:

  <xsl:template name="mytemplate">
    <xsl:param name="p"/>
    <xsl:call-template name="mytemplate_impl">
      <xsl:with-param name="p" select="$p"/>
    </xsl:call-template>
  </xsl:template>
  
  <xsl:template name="mytemplate_impl">
    <xsl:param name="p">default p</xsl:param>
    p = <xsl:value-of select="$p"/>
  </xsl:template>

When calling:
    <xsl:call-template name="mytemplate_impl"/>
the result is:
        p = default p
But when calling 
    <xsl:call-template name="mytemplate"/>
the result is:
        p =

what is wrong in my case, becase I did not supplied any value to parameter $p 
so I want to see its default value. I know that I can declare the same default 
value in template "mytemplate" , but then I will have the same value in more 
than one place and that's not good when I will need to change it.

Is there any workaround for my problem?

Thank you, 
Michal

PS: If you want to have full pricture why I need this default parameter 
mechanism, here it is:

I have hierarchy of importing stylesheets and I try to simulate polymorphism 
for named templates via shadowing of "interface" templates and by calling 
"implementation" templates.

library.xsl:
  <!-- interface template -->
  <xsl:template name="mytemplate">
    <xsl:param name="p"/>
    <xsl:call-template name="mytemplate_impl">
      <xsl:with-param name="p" select="$p"/>
    </xsl:call-template>
  </xsl:template>

  <!-- implementation template -->
  <xsl:template name="mytemplate_impl">
    <xsl:param name="p">default p</xsl:param>
    p = <xsl:value-of select="$p"/>
  </xsl:template>

customized-library.xsl:
  <xsl:import href="library.xsl"/>

  <!-- interface template -->
  <xsl:template name="mytemplate">
    <xsl:param name="p"/>
    <b>
      <xsl:call-template name="mytemplate_impl">
        <xsl:with-param name="p" select="$p"/>
      </xsl:call-template>
    </b>
  </xsl:template>

user-stylesheet.xsl:

  <xsl:include href="customized-library.xsl"/>
  ...
  <xsl:template match="/">
    <xsl:call-template name="mytemplate"/>
  </xsl:template>

user-input.xml:
<?xml-stylesheet type="text/xsl" href="user-stylesheet.xsl"?>
<root/>

required output:
  <b>p = default p</b>

actual output:
  <b>p = </b>

I hope the idea is ok, developer of user-stylesheet.xsl can customize templates 
from library.xsl that he/she uses in user-stylesheet.xsl. But I am having the 
problem with passing empty parameter (no parameter) that discard default value 
in original template.

-----------
Michal Ďurdina
ASSET Soft, a.s. / eBussiness Solutions
Košická 56, 821 08 Bratislava SLOVAK REPUBLIC
Phone: +421-2-50703-523
Fax: +421-2-50703-444
e-mail: michal(_dot_)durdina(_at_)assetsoft(_dot_)sk

http://www.assetsoft.sk