Do not place IO BELs at DUMMY tiles.

libtrellis/src/Chip.cpp

diff --git a/libtrellis/src/Chip.cpp b/libtrellis/src/Chip.cpp
index 339da74..60341f7 100644
--- a/libtrellis/src/Chip.cpp
+++ b/libtrellis/src/Chip.cpp
@@ -291,10 +291,10 @@
                 MachXO2Bels::add_lc(*rg, x, y, z);
 
         // PIO Bels
-        if (tile->info.type.find("PIC_L0") != string::npos || tile->info.type.find("PIC_LS0") != string::npos ||
-            tile->info.type.find("PIC_T") != string::npos ||
+        if (tile->info.type.find("DUMMY") == string::npos && (tile->info.type.find("PIC_L0") != string::npos ||
+            tile->info.type.find("PIC_LS0") != string::npos || tile->info.type.find("PIC_T") != string::npos ||
             tile->info.type.find("PIC_R0") != string::npos || tile->info.type.find("PIC_RS0") != string::npos ||
-            tile->info.type.find("PIC_B") != string::npos)
+            tile->info.type.find("PIC_B") != string::npos))
             for (int z = 0; z < 4; z++)
                 MachXO2Bels::add_pio(*rg, x, y, z);