Deductions can be made in this order:  f2=a1, and f2=e3, and the only symbol possible in all three cells is B.  Then c4=B, d5=B.  f1=e2=A, b3=A, d4=A, c6=A.  d3=D, a2=D, b4=D, f5=D, e6=D. a4=F, f4=C, f3=E, c3=F, a3=C, a6=E, d6=C, b5=E, e5=F, c5=C, etc.