let R be RelStr ; :: thesis: ( R is well_founded iff well_founded-Part R = the carrier of R )
set r = the InternalRel of R;
set c = the carrier of R;
set wfp = well_founded-Part R;
set IT = { S where S is Subset of R : ( S is well_founded & S is lower ) } ;
the carrier of R c= the carrier of R ;
then reconsider cs = the carrier of R as Subset of R ;
A1: well_founded-Part R = union { S where S is Subset of R : ( S is well_founded & S is lower ) } by Def4;
hereby :: thesis: ( well_founded-Part R = the carrier of R implies R is well_founded ) end;
assume A3: well_founded-Part R = the carrier of R ; :: thesis: R is well_founded
let Y be set ; :: according to WELLORD1:def 3,WELLFND1:def 2 :: thesis: ( not Y c= the carrier of R or Y = {} or ex b1 being object st
( b1 in Y & the InternalRel of R -Seg b1 misses Y ) )

assume that
A4: Y c= the carrier of R and
A5: Y <> {} ; :: thesis: ex b1 being object st
( b1 in Y & the InternalRel of R -Seg b1 misses Y )

consider y being object such that
A6: y in Y by ;
consider YY being set such that
A7: y in YY and
A8: YY in { S where S is Subset of R : ( S is well_founded & S is lower ) } by ;
consider S being Subset of R such that
A9: YY = S and
A10: ( S is well_founded & S is lower ) by A8;
set YS = Y /\ S;
A11: the InternalRel of R is_well_founded_in S by A10;
( Y /\ S c= S & Y /\ S <> {} ) by ;
then consider a being object such that
A12: a in Y /\ S and
A13: the InternalRel of R -Seg a misses Y /\ S by A11;
A14: a in Y by ;
a in S by ;
then A15: ( the InternalRel of R -Seg a) /\ Y = (( the InternalRel of R -Seg a) /\ S) /\ Y by
.= ( the InternalRel of R -Seg a) /\ (Y /\ S) by XBOOLE_1:16 ;
( the InternalRel of R -Seg a) /\ (Y /\ S) = {} by A13;
then the InternalRel of R -Seg a misses Y by A15;
hence ex b1 being object st
( b1 in Y & the InternalRel of R -Seg b1 misses Y ) by A14; :: thesis: verum