How do I add new instructions or code to an existing callback in a gui. I have a button that when I press in a GUI it performs a PCA for me. However I want to add Procrustes code so that any time I press that particular button in the GUI, PCA will be performed in addition to a Procrustes analysis. How do I add the Procrustes code to that part of the PCA code so that they communicate? Any code example?