expires.instrument: instrument expires extraction method
In redmode/FinancialInstrument: Financial Instrument Model Infrastructure for R

Returns either the last expiration date before Date, or the first expiration date after Date (if expired==FALSE).

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  ## S3 method for class 'instrument'
 expires(x, Date, expired = TRUE,
    silent = FALSE, ...)

`Date`	Can be a Date or character string. When `expires` is a vector, the retuned value will be one of the two values of `expires` that are closest to `Date`. (which one will be determined by the value of `expired`)
`expired`	TRUE/FALSE. This determines which date will be used when `expires` is a vector. If `expired` is `TRUE` the date returned will be the last one before `Date`. If `expired` is `FALSE` the first one after `Date` will be returned. Note that if `expires` is a single value, `expired` will be ignored.
`silent`	silence warnings?