Why this is not a correct definition for ordered triple? (I know that the correct definition is Kuratowski's definition)
for any $a,b,c$ : $<a,b,c> = \{ \{a\}, \{a, b\}, \{a, b, c\} \}$
At the first time I saw the definition of ordered pair I thougt this is correct but according to kuratowski's definition it is not and I don't know why.