let a, b be Real; :: thesis: for X being Subset of R^1 st X = ].a,b.] holds
Int X = ].a,b.[
let X be Subset of R^1; :: thesis: ( X = ].a,b.] implies Int X = ].a,b.[ )
assume A1: X = ].a,b.] ; :: thesis: Int X = ].a,b.[
A2: Int X c= X by TOPS_1:16;
thus Int X c= ].a,b.[ :: according to XBOOLE_0:def 10 :: thesis: ].a,b.[ c= Int X

proof

let x be object ; :: according to TARSKI:def 3 :: thesis: ( not x in Int X or x in ].a,b.[ )
assume A3: x in Int X ; :: thesis: x in ].a,b.[
then reconsider x = x as Point of R^1 ;

A4: now :: thesis: not x = b

now :: thesis: not a >= b

assume a >= b ; :: thesis: contradiction
then X = {} R^1 by A1, XXREAL_1:26;
hence contradiction by A3; :: thesis: verum

end;

then Fr X = {a,b} by A1, Th35;
then A5: ( Int X misses Fr X & b in Fr X ) by TARSKI:def 2, TOPS_1:39;
assume x = b ; :: thesis: contradiction
hence contradiction by A3, A5, XBOOLE_0:3; :: thesis: verum

end;

x <= b by A1, A2, A3, XXREAL_1:2;
then A6: x < b by A4, XXREAL_0:1;
a < x by A1, A2, A3, XXREAL_1:2;
hence x in ].a,b.[ by A6, XXREAL_1:4; :: thesis: verum

end;

reconsider Y = ].a,b.[ as open Subset of R^1 by BORSUK_5:39, TOPMETR:17;
Y c= Int X by A1, TOPS_1:24, XXREAL_1:41;
hence ].a,b.[ c= Int X ; :: thesis: verum