DuckCorp Dico

Found one definition

1.                 From The Jargon File (version 4.4.7, 29 Dec 2003):
                   
   
   
                   
                     
   flatten
    vt.
       [common] To remove structural information, esp. to filter something with an
       implicit tree structure into a simple sequence of leaves; also tends to
       imply mapping to flat-ASCII. “This code flattens an expression with
       parentheses into an equivalent canonical form.”