I think you are trying to say a "but" operator is equivalent "not and" operator and that !(A>B) is equivalent to "but".
I am assuming that "!" symbolizes the negation operator and ">" symbolized the ifthen ( or implies ) operator.
Yet ...
P  Q  P and Q  !(P and Q) 
T  T  T  F 
T  F  F  T 
F  T  F  T 
F  F  F  T 
So I would say "A and !B"is not equivalent to "!(A > B)" and I wouldn't say "A but B" <=> "A and !B", but I could just be misunderstanding your notation. :)
As far a what a but operator is I would say it should be equivalent to the "Boolean And" operator.
And is not needed in a programming language. The word "but" is used in commonly to emphasis
that an assumption is false. Example ...
If a student where to errorously assume that multiplaction is
the same as addition the student might state ...
1 + 1 = 2 and 2 + 2 = 4 and 2 * 2 = 4 and 1 * 1 = 2
... which we know to be false. The student's teacher would say ...
1 + 1 = 2 and 2 + 2 = 4 and 2 * 2 = 4 but 1 * 1 != 2
... to correct the student.
This is the same as
1 + 1 = 2 and 2 + 2 = 4 and 2 * 2 = 4 and 1 * 1 != 2
T and T and T and T is TRUE
It is not equal to
1 + 1 = 2 and 2 + 2 = 4 and 2 * 2 = 4 and not (1 * 1 != 2)
T and T and T and F is FALSE
But merely gives emphasis to the fact that the student's assumption is false.
Janitored by davido: removed excess spaces within pre tags that caused horizontal scrolling.
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