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Nondeterministic selections are delimited by : in HAC, whereas they were delimited by | in CAST.
In CAST, send and receive actions were written as: `X!x' or `Y!z', but in HAC, send and receive arguments must be enclosed within parenthesis like function call arguments: `X!(x)' or `Y!(z)'. Rationale: syntactic consistency and disambiguation with the concurrency operator (comma).
HAC supports receive actions that do not write to any variables. `Y?' acknowledges channel Y without writing its values.