Add information on trapping work-flow README

build/README.md

@@ -62,6 +62,6 @@ The default behaviour of `np_sphere` is to take the same input files as `edfb` a
 
 ### trapping
 
-The execution of trapping programs requires at least one of the previous programs to have produced a complete output set.
+The execution of trapping programs requires at least one of the previous programs to have produced a complete output set. The working assumption is that the `frfme` program is a pre-requisite to `lffft`.
 
 *TODO:* Define a common format for binary I/O operations on the TTMS file.