Running arachne on counter.v from tutorial 4 (the 26 bit counter) results in a fatal error due to missing set_io constraints. The counter.v file contains:

module counter(input clk, output [25:0] data);

while the .pcf contains only pin destinations for the first four bits:

set_io data[25] 96
set_io data[24] 97
set_io data[23] 98
set_io data[22] 99
set_io clk 21

Leading to arachne throwing up a fatal error: Missing 22 set_io constraints.