Including icons in Activity diagram in PlantUML?

Context

To improve my understanding on how SSH is used I drew a diagram in PlantUML using the Domain Story: Live Example

However, I think it looks a bit chaotic, hence I would like to convert it into a sequence diagram instead.

Error

When I try to import an icon into the sequence diagram as:

@startuml
actor Bob #red
' The only difference between actor
'and participant is the drawing
participant Alice
participant "I have a really\nlong name" as L #99FF99
/' You can also declare:
   participant L as "I have a really\nlong name"  #99FF99
  '/

' This is required to grant access to all icons.
!include <material/common>
' This imports the actual door icon.
!include <material/door>
MA_DOOR(Purple, 1, setup_public_key, rectangle, "Public key"){}


Alice->Bob: Authentication Request
Bob->Alice: Authentication Response
Bob->L: Log transaction
@enduml

it throws

rectangle "color:Purple<$ma_door{schale=1}>\r Public Key" as setup_public_key <<MA_DOOR>>{}

See Live Example

Question

How can I include the material/common icons like door, key and quiz etc. in a sequence diagram?

Code

@startuml
'java -jar plantuml.jar /home/name/git/bash/bash-ssh-over-tor/visual/usage.uml

' This allows the user to create stories.
!include https://raw.githubusercontent.com/johthor/DomainStory-PlantUML/main/domainStory.puml

' This is required to grant access to all icons.
!include <material/common>
' This imports the actual door icon.
!include <material/door>
!include <material/account_key>
!include <material/key>
!include <material/cancel>
'!include <material/quiz>
!include <material/help>
!include <material/thumb_up>

Boundary("Usage") {
    Person(Alice)
    System(Leader)
    System(Follower)
    Boundary("keypair") {

        ' This creates a purple door icon.
        MA_DOOR(Purple, 1, setup_public_key, rectangle, "Public key")
        MA_KEY(Green, 1, setup_private_key, rectangle, "Private key")
    }
    ' This creates a green door. The rectangle places the icon in a rectangle.
    MA_ACCOUNT_KEY(Green, 1, setup_authorized_key, rectangle, "Authorised Public Key:\n any user that proves it has a\n private key belonging to this public key\n can have access.")

    MA_KEY(Green, 1, same_setup_private_key, rectangle, "Private key")
    MA_DOOR(Purple, 1, same_setup_public_key, rectangle, "Public key")

    MA_THUMB_UP(green, 1, solution, rectangle, "Solution")
    MA_HELP(Orange, 1, challenge, rectangle, "Challenge")
    MA_CANCEL(Red, 1, same_setup_cancel, rectangle, "This Public Key is not authorised.")
    MA_CANCEL(Red, 1, solution_rejection, rectangle, "This solution to the challenge\n was invalid or did not match the authorized key.")
}


activity(l, Leader, "Has SSH access into Follower", Follower)
activity(k, setup_authorized_key, "If the solution is valid\n and matches the Authorized\n Public key, SSH access is\n granted to the Leader", Leader)
activity(j, setup_authorized_key, "If the solution is invalid\n or does not match the Authorized Public key,\n SSH access is rejected", solution_rejection)
activity(i, Follower, "checks that the solution", solution, " is valid and matches the Authorised Public Key ", setup_authorized_key)
activity(i, solution, "and provided to ", Follower)
activity(h, challenge, "is solved", solution)
activity(g, Leader, "takes private key", setup_private_key, "and solves the challenge", challenge)
activity(f, challenge, to, Leader)
activity(e, setup_authorized_key, "If the public key is authorized\n, give a challenge", challenge)
activity(d, setup_authorized_key, "If the public key is not authorized, decline access.\n", same_setup_cancel)
activity(c, Follower, "checks is that public key: ", same_setup_public_key, "an authorised public key?", setup_authorized_key)
activity(b, Leader, "and asks Follower:\n ''Can I SSH access you with this Public Key?''", setup_public_key, to , Follower)
activity(a, Alice, "on the Leader machine", Leader)
@enduml

When I try to import an icon into the sequence diagram as: ... it throws:

The import is not the source of the error. The problem is with the line of the macro (MA_DOOR) that creates an element (rectangle) which has {} at the end. A rectangle is not compatible with the sequence diagram notation, and the {} at the end is a syntax error.

The solution is either to use the MA_DOOR macro with a participant argument and remove the {} at the end, or to use the sprites (<$ma_door{scale=1}>) directly in an element name.

@startuml
' Style the entity (see https://github.com/plantuml/plantuml-stdlib/blob/master/material/door.puml)
hide <<MA DOOR>> stereotype
<style>
.MA DOOR {
    BackgroundColor LightGreen
}
</style>

actor Bob #red
!include <material/common>
!include <material/door>
' the following macro generates an entity of type participant and doesn't have {} at the end
' see https://github.com/plantuml/plantuml-stdlib/blob/master/material/door.puml for how the macro is defined
MA_DOOR(Purple, 1, setup_public_key, participant, "Public key")

' optionally use the $ma_door sprite inside a participant (or actor)
participant "<color:blue><$ma_door{scale=1}>\nDoor" as door

Bob -> setup_public_key : open
Bob -> door : open
@enduml