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Syntax
  1. constraint << Symbol
  2. constraint !<< Symbol
where a constraint is any character class, a literal or a keyword non-terminal Symbol.
Description Using <<, the parser will not accept the Symbol if it is not immediately preceded by the terminal in the input string. If the start of the symbol coincides with start of the inout, the constraint will fail and the symbol will not be accepted.

Using !<<, the parser will not accept the Symbol if it is immediately preceded by the terminal in the input string. If the start of the symbol coincides with start of the inout, the constraint will always succeed and the symbol is accepted.
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