Use chformal to remove unreachable cover cells

vai_fifo/symbiyosys.sby

@@ -17,7 +17,13 @@ prove: abc pdr
 [script]
 ghdl --std=08 -gFormal=true -gDepth=16 -gWidth=16 fifo.vhd vai_fifo.vhd -e vai_fifo
 prep -top vai_fifo
+# Convert all assumes to asserts in sub-unit i_fifo
 chformal -assume2assert vai_fifo/i_fifo %M
+# Remove selected covers in i_fifo sub-unit as they cannot be reached
+chformal -cover -remove */formalg.read_pnt_stable_when_empty.cover
+chformal -cover -remove */formalg.rerror.cover
+chformal -cover -remove */formalg.werror.cover
+chformal -cover -remove */formalg.write_pnt_stable_when_full.cover
 
 [files]
 ../fifo/fifo.vhd