Fórum Erro de Compilação: Left side cannot be assigned #330642

Este eo Código que gera o erro

const
Contexts : TContextSet = [];

procedure AddContext( ct : TContextType );
begin
[color=red:cc073c3289] Contexts := Contexts + [ ct ];[/color:cc073c3289]
end;

procedure RemoveContext( ct : TContextType );
begin
[color=red:cc073c3289] Contexts := Contexts - [ ct ];[/color:cc073c3289]
end;

procedure ClearContexts;
begin
[color=red:cc073c3289] Contexts := [];[/color:cc073c3289]
end;

Este e o erro de Copilação

[Error] HTMLReader.pas(237): Left side cannot be assigned to
[Error] HTMLReader.pas(241): Left side cannot be assigned to
[Error] HTMLReader.pas(245): Left side cannot be assigned to

desde já Agadeço!