Suppose I define a variable like this:

<dfdl:defineVariable name="boundary" defaultValue="some_boundary" type="xsd:string"/>

Can I later concatenate it with other values? 

For example, can I concatenate it as part of a pattern?

<xsd:element name="Preamble" type="xsd:string" dfdl:lengthKind="pattern" dfdl:lengthPattern="(.+?)(?=--{$some_boundary})"/>

Can I concatenate it as part of a separator?

<xsd:sequence dfdl:separator="--{$some_boundary}" dfdl:separatorSuppressionPolicy="anyEmpty"  dfdl:terminator="--%CR;%LF;">

TIA!