Any sequence of drawables may be grouped in a block merely by
enclosing them in braces { }. A block is itself drawable. A
key use of blocks is to extend the effect of a single def,
Definitions, put Puts, sweep Sweeps,
or repeat Repeats to include several objects rather than
one.
Definitions (See Definitions.) inside a block have lexical scope extending from the place of definition to the end of the block.