You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: key.go
    path of file: ./repos/teller/vendor/github.com/gopasspw/gopass/internal/backend/crypto/gpg
    
    the code of the file until where you have to start completion: package gpg

import (
	"fmt"
	"sort"
	"time"
)

// Key is a GPG key (public or secret).
type
    
func (k Key) IsUseable(alwaysTrust bool) bool {
	if k.Caps.Deactivated {
		return false
	}

	if !k.Caps.Encrypt {
		return false
	}

	if !k.ExpirationDate.IsZero() && k.ExpirationDate.Before(time.Now()) {
		return false
	}

	if alwaysTrust {
		return true
	}

	switch k.Validity {
	case "m":
		return true
	case "f":
		return true
	case "u":
		return true
	}

	return false
}