Hi,

Just found out this edge case in the parser on v13.1.1

filter([10, 20], f(x, index) = index == 1)     # yields [10, 20]
# expected [10]
filter([10, 20], f(x, index) = index == 1000)  # yields [10, 20]
# expected []

Outside the parser there is no issue.

math.filter([10, 20], (x, index) => index == 0)    // yields [10]
math.filter([10, 20], (x, index) => index == 1000) // yields []

I found it while working on #3256, I'm still trying to find the source of the error.