let p be Point of (TOP-REAL 2); :: thesis: north_halfline p = { |[(p `1),r]| where r is Real : r >= p `2 }
set A = { |[(p `1),r]| where r is Real : r >= p `2 } ;
hereby :: according to TARSKI:def 3,XBOOLE_0:def 10 :: thesis: { |[(p `1),r]| where r is Real : r >= p `2 } c= north_halfline p
let x be object ; :: thesis: ( x in north_halfline p implies x in { |[(p `1),r]| where r is Real : r >= p `2 } )
assume A1: x in north_halfline p ; :: thesis: x in { |[(p `1),r]| where r is Real : r >= p `2 }
then reconsider q = x as Point of (TOP-REAL 2) ;
A2: q `2 >= p `2 by A1, Def10;
A3: q = |[(q `1),(q `2)]| by EUCLID:53;
q `1 = p `1 by A1, Def10;
hence x in { |[(p `1),r]| where r is Real : r >= p `2 } by A2, A3; :: thesis: verum
end;
let x be object ; :: according to TARSKI:def 3 :: thesis: ( not x in { |[(p `1),r]| where r is Real : r >= p `2 } or x in north_halfline p )
assume x in { |[(p `1),r]| where r is Real : r >= p `2 } ; :: thesis: x in north_halfline p
then consider r being Real such that
A4: x = |[(p `1),r]| and
A5: r >= p `2 ;
reconsider q = x as Point of (TOP-REAL 2) by A4;
A6: q `2 = r by A4, EUCLID:52;
q `1 = p `1 by A4, EUCLID:52;
hence x in north_halfline p by A5, A6, Def10; :: thesis: verum