Output signal in trap which caused the trap being called?

Question

Let's say I have the following trap, which catches some signals, which terminate the script:

#!/bin/sh
trapterm() {
    echo "Process (PID: $$) interrupted." >&2
    exit 2
}

# add trap to catch terminating signals
trap trapterm INT TERM
# […]

Now, how can I get the signal, which caused the trap being called, and output it? The result should work like this fictitious $SIGNAL:

#!/bin/sh
trapterm() {
    echo "Process (PID: $$) interrupted by signal $SIGNAL." >&2
    exit 2
}

# add trap to catch terminating signals
trap trapterm INT TERM
# […]

Shell-specific solutions (bash) are okay, but I'd prefer a POSIX-compatible ones.

Answer 1

As man 1p trap claims that the action (the first argument to trap) is simply evaled without further information about the signal recieved a portable solution might be to install different handlers for each signal:

#!/bin/sh
trap_sig() {
    echo "Process (PID: $$) interrupted by signal $1." >&2
    exit 2
}

# add trap to catch terminating signals
trap 'trap_sig INT' INT
trap 'trap_sig TERM' TERM
# […]