How to properly define a common interface across Ruby versions, engines and engine versions

shim.rb

# The goal is to define a common set of methods independent of Ruby version
# engine, and engine version to make other code more simple. The problem is:
#   1. the method may not be defined for the advertised Ruby version
#   2. the method may be defined but "shouldn't" be for that version
#   3. the method may not be defined and shouldn't be defined
# The conditional needs to account for these cases. If the method is not
# defined but should be for that Ruby version, that should be a hard error.
# If the method is defined but shouldn't be for that Ruby version, it may
# not be a compatible definition and that should be checked early rather
# than resulting in a confusing error later. Finally, if it's not defined
# and shouldn't be, it's probably safe to define it.
#
# NOTE: Extra special care must be taken when defining a method that is
# "not" defined (eg could be private) for that Ruby version in case it
# over-rides an already defined engine-specific method.
#
# Pseudo-ish code
if expected_version and not expected_method
  error "method expected for version"
elsif not expected_version and expected_method
  if not check_expect_method_is_compatible
    error "expected method is not compatible"
  end
else
  define_method
end