[Newbies] understanding selectors

[Jonathan Wright]

Hello,

I'm trying to extend the Matrix class by making it able to add and
subtract matrices.  So far I've implemented the following code to add:

+ aMatrix
	| newMatrix rowCount columnCount sum |
	rowCount := self rowCount.
	columnCount := self columnCount.
	newMatrix := Matrix rows: rowCount columns: columnCount.
	
	1 to: rowCount do: [ :selectRow | 
		1 to: columnCount do: [ :selectColumn |
		sum := (self at: selectRow at: selectColumn) + 
					(aMatrix at: selectRow at:
  selectColumn). newMatrix at: selectRow at: selectColumn put: sum.]].
	^newMatrix.

Now I want to implement a method for subtracting matrices.  However,
I'd like to use the same code.  I tried to implement an operand
selector, however, it errors out.

Something like this:

operand: operand matrix: aMatrix
	| newMatrix rowCount columnCount sum |
	rowCount := self rowCount.
	columnCount := self columnCount.
	newMatrix := Matrix rows: rowCount columns: columnCount.
	
	1 to: rowCount do: [ :selectRow | 
		1 to: columnCount do: [ :selectColumn |
		sum := (self at: selectRow at: selectColumn) operand 
					(aMatrix at: selectRow at:
  selectColumn). newMatrix at: selectRow at: selectColumn put: sum.]].
	^newMatrix.

I know this is not SmallTalk convention, but how should I pursue
something like this?

Thank You,
Jonathan