A sequence of more than one item is not allowed as the

I fear this may be a FAQ...

<xsl:variable name="foo" as="xs:string">
  <xsl:text/>abc<xsl:value-of select="'def'"/>
</xsl:variable>

<xsl:value-of select="$foo"/>

Gives the error message: "A sequence of more than one item is not
allowed as the value of item $foo"

This is because variable $foo contains two strings, yet I've defined it
as type xs:string - which it seems refers to a single string.  If I
define it as xs:string+ I get the separator and then need to use
string-join().

If I drop the as="xs:string" attribute I get the desired result 'abcdef'
- but this is because that's the string value of the nodeset.

This is very upside down for me.

Is it really the case that we will have to be very aware of how many
items we are creating and construct large concat()'s, or have a liberal
sprinking string-join()'s throughout the stylesheet?  Or am I missing
something and I shouldn't be using the type xs:string in this way?

cheers
andrew