RE: how to match the absence of a particular attribute

how can I test the presence of a particular attribute of a node in an
"if" or "choose" tag?
I need to drive the behavior of the translation in two different ways,
if an attribute exists or not.

<A>
  <B a="a">hello2</B>
  <B a="a" b="b">hello3</B>
  <B>hello1</B>
  <B b="b">hello4</B>
</A>

<xsl:template match="A/B[@a or @b]">
   <xsl:copy-of select="."/>
</xsl:template>

I suppose the key point is the
capability to match the absence of an attribute, but I have found no
way to do this.