Note 6 - Shareholders' Equity - Common Stock Reserved for Future Issuance (Details) - shares	Mar. 31, 2022	Dec. 31, 2021
Stock options outstanding (in shares)	1,625,097	1,664,014
Stock options and restricted stock units available for future grant (in shares)	1,045,772
Common stock reserved for future issuance (in shares)	3,104,843
Restricted Stock Units and Performance Stock Units [Member]
Restricted stock units and performance stock units outstanding (in shares)	433,974

Aggregate number of common shares reserved for future issuance.

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

+ References

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-Publisher FASB
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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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Number of options outstanding, including both vested and non-vested options.

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