Cynthia Yeh, principal percussionist for the Chicago Symphony Orchestra, talks about her preparations — including the appropriate footwear! — for the concerto Veni, Veni, Emmanuel, in which she will be the featured soloist in performances Dec. 18-20. Since she performs throughout the percussion concerto, “I had to choreograph where I was going to put everything,” she says, “and lots of things someone would normally not have to think about.”