[DFDL-WG] Can I concatenate a variable?

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!