Function decode_with_codec 

pub fn decode_with_codec(
    cx: &mut Cx,
    symbol: &Symbol,
    input: Input,
    read_policy: ReadPolicy,
) -> Result<Expr>

Decode input with the codec named symbol, using default DecodeLimits.

Looks the codec up in the kernel registry, runs its decoder under read_policy, and returns the raw Expr.