Errors and mismatches due to videos being from an earlier semester:

• A video at one point says “step_continue always sets the mode to interp_mode”, but that’s wrong. A step_continue method sets the mode to either interp_mode or continue_mode.
• The videos and slides mention and optimize method and mode, which we don't have this year. This year, your MSDscript implementations have print and pretty_print, instead.
• CompExpr is an olded name for EqExpr.
• The arith4 program run on the command line is supposed to be msdscript.