The use of "process_label" at the beginning and end of a process is
optional but recommended to enhance code readability.
The "sensitivity_list" is optional in that a process may have either a
sensitivity_list, or it must include "wait" statements. A process
cannot include both a sensitivity_list and wait statements. Wait
statements will be covered in a subsequent section.
The "process_declaration" includes declarations for variables, constants,
aliases, files, and many other things.
The "process_statements" include variable assignment statements,
signal assignment statements, procedure calls, wait statements, "if"
clauses, "while" loops, assertion statements, etc.
