def declaration_files(decl_or_decls):
files = set()<EOL>decls = make_flatten(decl_or_decls)<EOL>for decl in decls:<EOL><INDENT>if decl.location:<EOL><INDENT>files.add(decl.location.file_name)<EOL><DEDENT><DEDENT>return files<EOL>
Returns set of files

Every declaration is declared in some file. This function returns set, that
contains all file names of declarations.

:param decl_or_decls: reference to list of declaration's or single
    declaration
:type decl_or_decls: :class:`declaration_t` or [:class:`declaration_t`]
:rtype: set(declaration file names)