Provenance gives Tuple not found result for imported facts with symbol type.

[zweiyi21]

.decl edge(x:symbol, y:symbol)
.input edge

.decl path(x:symbol, y:symbol)
.output path

path(x, y) :- edge(x, y).
path(x, y) :- path(x, z), edge(z, y).

I slightly modified the tutorial example to change the data type from number to symbol, if the base fact edge is imported from a file, the explain will give a Tuple not found result.

souffle -F. -D. -t explain example.dl
Explain is invoked.
Enter command > explain path("2","3")
Tuple not found
Enter command > 

I am sure that the edge.facts file is in the correct path and the contents of the edge.facts are

"1"	"2"
"2"	"3"

Note that, if the base fact is contained in the example.dl, it works fine. And for the number type, no such problem for the imported facts.

.decl edge(x:symbol, y:symbol)
edge("1", "2").
edge("2", "3").

.decl path(x:symbol, y:symbol)
.output path

path(x, y) :- edge(x, y).
path(x, y) :- path(x, z), edge(z, y).

souffle -D. -t explain example.dl
Explain is invoked.
Enter command > explain path("2","3")
edge("2", "3") 
-----------(R1)
path("2", "3") 

The version info

souffle --version
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