What is the meaning of ``separateExpression`` and ``isSeparable``?

[SergeStinckwich]

The semantic of the separateExpression method on KEBinaryExpression is not really clear.
Tests should give the semantic more clearly and the method should be renamed appropriately.

testSeparateABinaryExpression

	| s1 s2 s3 s4 |
	s1 := 's=-(A+B+C)*E*D' parseAsAnEquation.
	s2 := 's=-(A+B)*(-C)' parseAsAnEquation.
	s3 := 's=(-E)*D*F*(A+B)' parseAsAnEquation.
	s4 := 's=(A+B)*(C+D)' parseAsAnEquation.
	self assert: 3 equals: s1 expression separateExpression size.
	self assert: 2 equals: s2 expression separateExpression size.
	self assert: 2 equals: s3 expression separateExpression size.
	self assert: 4 equals: s4 expression separateExpression size

The same for the method isSeparable.

[SergeStinckwich]

separateExpression is used only in generateEvents method on KEExpression.
After a while, I was able to understand that separateExpression is about being able to express an expression as a sum of factors (the reverse of the factorization).

We need to replace this method with an expand method on KEExpression.

[mikalziane]

the form of the test is not appropriate. One should be able to express 2 different things in tests
a) expr 1 = expr 2
b) f(expr1) == expr 2 where == means identical in form

[SergeStinckwich]

I refactor the test as:

testSeparateABinaryExpression

	| e1 e2 e3 e4 |
	e1 := '-(A+B+C)*E*D' parseAsAnExpression.
	e2 := '-(A+B)*(-C)' parseAsAnExpression.
	e3 := '(-E)*D*F*(A+B)' parseAsAnExpression.
	e4 := '(A+B)*(C+D)' parseAsAnExpression.
	self assert: e1 separateExpression size equals: 3.
	self assert: e2 separateExpression size equals: 2.
	self assert: e3 separateExpression size equals: 2.
	self assert: e4 separateExpression size equals: 4

More refactorings are needed in order to remove the use of Strings.