Casting while grappled: Let's settle this

[entitivity]

GURPS Campaigns says this of attempting a concentrate maneuver while grappled:

If you have been grappled...Aim, Feint, Concentrate, and Wait maneuvers – and ranged attacks – are completely impossible. (B371)

GURPS Magic says this of casting while being grappled:

If you use an active defense against an attack, or are knocked back, knocked down, injured, grappled, or otherwise distracted while concentrating, make a Will roll at -3 to continue casting your spell. On a failure, your spell is spoiled and you must start over. (M7) (Emphasis mine)

I take the above to mean that if you are grappled, you cannot begin a concentrate manuever, but if you are already concentrating and someone grapples you, you can attempt to finish what you started. Is this correct?