Block
 Usage
• Block[{x, y, ... }, expr] specifies that expr is to be evaluated with local values for the symbols x, y, ... .
• Block[{x =  , ... }, expr] defines initial local values for x, ... .
Notes
• Block allows you to set up an environment in which the values of variables can temporarily be changed. • When you execute a block, values assigned to x, y, ... are cleared. When the execution of the block is finished, the original values of these symbols are restored. • Block affects only the values of symbols, not their names. • Initial values specified for x, y, ... are evaluated before x, y, ... are cleared. • You can use Block[{vars}, body /; cond] as the right-hand side of a transformation rule with a condition attached. • Block implements dynamic scoping of variables. • Block is automatically used to localize values of iterators in iteration constructs such as Do, Sum and Table. • New in Version 1.
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