Panic: attempt to subtract with overflow

$ RUST_BACKTRACE=1 cargo run
   Compiling proost v0.1.0 (/home/jean/repos/proost/proost)
    Finished dev [unoptimized + debuginfo] target(s) in 2.11s
     Running `target/debug/proost`
Welcome to proost 0.1.0
» def And: Prop -> Prop -> Prop := fun A B: Prop => (C: Prop) -> (A -> B -> C) -> C

✓
» 
def and_comm: (A B: Prop) -> And A B -> And B A :=
  fun A B: Prop, f: And A B, C: Prop, bac: B -> A -> C => f (fun a:A, b:B => bac b a)

thread 'main' panicked at 'attempt to subtract with overflow', proost/src/main.rs:162:90
stack backtrace:
   0: rust_begin_unwind
             at /rustc/23415176968e81e0aac92d0218612a89c4e68a82/library/std/src/panicking.rs:575:5
   1: core::panicking::panic_fmt
             at /rustc/23415176968e81e0aac92d0218612a89c4e68a82/library/core/src/panicking.rs:64:14
   2: core::panicking::panic
             at /rustc/23415176968e81e0aac92d0218612a89c4e68a82/library/core/src/panicking.rs:111:5
   3: proost::display
             at ./proost/src/main.rs:162:90
   4: proost::main::{{closure}}
             at ./proost/src/main.rs:125:21
   5: kernel::memory::arena::use_arena
             at ./kernel/src/memory/arena.rs:74:5
   6: proost::main
             at ./proost/src/main.rs:113:5
   7: core::ops::function::FnOnce::call_once
             at /rustc/23415176968e81e0aac92d0218612a89c4e68a82/library/core/src/ops/function.rs:507:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

A dbg! call shows

[proost/src/main.rs:158] loc.start.column = 59
[proost/src/main.rs:158] loc.end.column = 1

and therefore the expression

loc.end.column - loc.start.column - 1

gets negative.