Paste: multi part confusion

TUPLE: part 
parent fields attributes data ;

TUPLE: multipart 
    fields attributes
    { mime-parts vector }
    ;

: add ( part multipart -- )
    [ swap parent<< ] 
    [ get-boundary set-boundary drop ]  ! assign the boundary to child.
    [ mime-parts>> push ] 2tri ;         ! add the part to the multipart parent.