Using removeVariable: in some case modify the object that receive the message

[SergeStinckwich]

The following test for example is currently false:

testRemoveAVariableShouldNotModifyTheExpression

	| e n |
	e := KEUnaryExpression new
		     op: #-;
		     expression: (KEVariable new symbol: #S).
	n := e removeVariable: (KEVariable new symbol: #S).
	self assert: e expression isNil equals: false```