(Warning) shift creation: left < right

[LEARNTAU]

Tau version: 0.7 (v0.7-alpha-145-g82e48cd)
Ubuntu Linux OS

Hello, I tried to play with the code provided by ptt kk shown in the Tau dev chat.

input

succ(int0) := int1
succ(int1) := int2
succ(int2) := int3
succ(int3) := int4
succ(int4) := int5
pred(intt) := intt-1
add(x, int0) := x
add(int0, x) := x
add(x, y) := add(pred(x), succ(y))
mul(x, int1) := x
mul(int1, x) := x
mul(x, y) := add(y, mul(pred(x), y))

Everything seems to work properly, but when I entered

n mul(int0,int2)

I got this non-stop message: (Warning) shift creation: left < right

I cant's stop the message from appearing continuously. Is this a bug?

[pt7k]

Good catch. It goes into some sort of endless loop due to an unhandled edge case in the multiplication recurrence.
On tau version (v0.7-alpha-136-g6df5e78) it would just go on forever without any warning messages, but if you "set severity debug" then it spits out the ever looping recurrence and crashes after a while.
Obviously the edge case can be easily fixed for the multiplication example by adding the following lines before the mul(x, y) definition's line:

mul(x, int[0](1)) := int[0](1)
mul(int[0](1), x) := int[0](1)

but still this example shows that an endless loop can be created while using recurrence functions.