The package provides many (purely expandable) tools for LaTeX:
Extensive list management (csv lists, lists of single
tokens/characters, etoolbox lists); purely expandable loops
(csvloop, forcsvloop, etc.); conversion (csvtolist, etc.));
addition/deletion (csvadd, listdel, etc.); Expansion and group
control: \expandnext, \ExpandAfterCmds, \AfterGroup...; Tests
on tokens, characters and control sequences (\iffirstchar,
\ifiscs, \ifdefcount, \@ifchar...); Tests on strings
(\ifstrnum, \ifuppercase, \DeclareStringFilter...); Purely
expandable macros with options (\FE@testopt, \FE@ifstar) or
modifiers (\FE@modifiers); Some purely expandable numerics
(\interval, \locinterplin).