# fn:trace - XPath XQuery Reference

Provides an execution trace intended to be used in debugging queries.deterministiccontext-independentfocus-independentThe function returns the value of $value, unchanged.In addition, the values of $value, converted to an xs:string,
            and $label(if supplied)may be directed to a trace data set. The destination of the trace
            output is implementation-defined. The format of the trace output is
            implementation-dependent. The ordering of output from calls of the
            fn:trace function is implementation-dependent.Consider a situation in which a user wants to investigate the actual value passed to
               a function. Assume that in a particular execution, $v is an
                  xs:decimal with value 124.84. Writing fn:trace($v,
                  'the value of $v is:') will put the strings "124.84" and
                  "the value of $v is:" in the trace data set in implementation
               dependent order.Sometimes there is a need to output trace information unrelated to a specific value.
            In such cases it can be useful to set $value to an empty string or an empty
            sequence, and to compute the value of the $label argument: 
            fn:trace((), "Processing item " || $i).