What does the [] operator do here?

[Net]

from the below code, the expression "case [c]":

void main()
{
     import std.stdio, std.string, std.algorithm, std.conv;

     // Reduce the RPN expression using a stack
     readln.split.fold!((stack, op)
     {
         switch (op)
         {
             // Generate operator switch cases statically
             static foreach (c; "+-*/")
                 case [c]:
                     return stack[0 .. $ - 2] ~
                         mixin("stack[$ - 2] " ~ c ~
                             " stack[$ - 1]");
             default: return stack ~ op.to!real;
         }
     })((real[]).init).writeln;
}