Boolean documentation

# Boolean

Boolean inherits from Object and defines no instance variables. It has subclasses False and True.

To save space, most control flow messages are not implemented in class Boolean---instead, True and False implement them directly.

## Methods defined in Boolean:

### Methods for copying:

- shallowCopy
  Copying Boolean values is not allowed.

### Methods for error handling:

- mustBeBoolean
  Sent by the VM if the receiver to an #ifTrue: or related message is not true or false.

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