Provided by: systemtap-doc_2.3-1ubuntu1.4_all 

NAME
function::is_return - Whether the current probe context is a return probe
SYNOPSIS
is_return:long()
ARGUMENTS
None
DESCRIPTION
Returns 1 if the current probe context is a return probe, returns 0 otherwise.
SystemTap Tapset Reference March 2016 FUNCTION::IS_RETURN(3stap)