Blocks are used both to define a hierarchy within a design and to
group together signal assignments which are only to be evaluated,
and, in fact potentially connected, when the defined GUARD is true.
The primary reason to organize an architecture as several blocks of
activity is to control the actions of groups of concurrent statements.
A conditional GUARD can be stated at the head of a block. If such a
condition expression exists, then any statement in the block which has
the keyword guarded will disconnect if the expression evaluates
to false.
