Changes

* turtle: specify which turtle is active

* turtle: specify which turtle is active

* turtle shell: import a image from the Journal to use as the turtle's 'shell', i.e., replace the turtle with a sprite.

* turtle shell: import a image from the Journal to use as the turtle's 'shell', i.e., replace the turtle with a sprite.

* top of stack: block to denote the top of a collapsible stack

* sandwich clamp: "clamp" a stack of blocks to hide

* bottom of stack: block to denote the bottom of a collapsible stack

:: To use the collapsible-stack blocks, place them around the blocks you want to hide

====Portfolio Palette====

====Portfolio Palette====