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|context> => |context: GBP USD exchange rate>

supported-ops |currency: USD: *> => |op: to-USD> + |op: to-GBP>
to-USD |currency: USD: *> #=> |_self>
to-GBP |currency: USD: *> #=> merge-labels(|currency: GBP: > + divide-by[1.43] extract-value |_self>)

supported-ops |currency: GBP: *> => |op: to-GBP> + |op: to-USD>
to-GBP |currency: GBP: *> #=> |_self>
to-USD |currency: GBP: *> #=> merge-labels(|currency: USD: > + times-by[1.43] extract-value |_self>)
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