Deductions can be made in this order:  a1=b5, and can only be E or F (not C because of c3); but if a1=E, noplace for E in the bottom right region; so a1=b5=F.  c1=B, b6=C, f1=E, b3=E, b2=D, b1=A, e1=C, e5=D, c5=E, d5=B, f5=C, a6=B, f2=B, e3=B, d4=C, a2=C, d3=F, c2=F, e4=F, e6=A, etc.