This is growing in my rock garden, quite obviously on purpose, but do I remember what it is? Of course not. It looks like squill to me, but I don't have any record of putting a spring-blooming squill in the rock garden. I did grow Scilla litardieri from seed a few years ago - the resulting patch is still living in my nursery area (and not blooming), but it is possible that I moved a couple of them to the rock garden last year and forgot to write it down. S. litardieri, in photos I've found, looks similar but perhaps not quite the same as the plant in my photo. Any better ideas?