Create a case-insensitive startsWith

[PhilipDaniels via Digitalmars-d-learn]

Beginner question. Given

   if (startsWith(input, "0x", "0X"))

How do I turn that into a case-insensitive startsWith? startsWith 
says it takes a predicate but I can't figure out how to pass it 
one. The examples all use "a == b" !? These attempts below, and 
other things I have tried, fail with "cannot deduce function from 
argument types".

   if (startsWith!"icmp(a, b) == 0"(input, "0x"))
   if (startsWith!"std.uni.icmp(a, b) == 0"(input, "0x"))
   if (startsWith!((a,b) => icmp(a,b) == 0)(input, "0x"))