Suppose you have constructed a sequence of attributes.

How do you access a value of attribute "a"?

Simple, isn't it? It has taken a couple of minutes to find a solution!

<xsl:variable name="attributes" as="attribute()*">
  <xsl:apply-templates mode="t:generate-attributes" select="."/>
</xsl:variable>

<xsl:variable name="value" as="xs:string?"
  select="$attributes[self::attribute(a)]"/>