add (:) meta-predicate argument indicator (#1302)

src/machine/loader.rs

@@ -2200,6 +2200,7 @@ impl Machine {
                         MetaSpec::Minus => atom_as_cell!(atom!("+")),
                         MetaSpec::Plus => atom_as_cell!(atom!("-")),
                         MetaSpec::Either => atom_as_cell!(atom!("?")),
+                        MetaSpec::Colon => atom_as_cell!(atom!(":")),
                         MetaSpec::RequiresExpansionWithArgument(ref arg_num) => {
                             fixnum_as_cell!(Fixnum::build_with(*arg_num as i64))
                         }

src/machine/preprocessor.rs

@@ -296,6 +296,7 @@ fn setup_meta_predicate<'a, LS: LoadState<'a>>(
                         atom!("+") => MetaSpec::Plus,
                         atom!("-") => MetaSpec::Minus,
                         atom!("?") => MetaSpec::Either,
+                        atom!(":") => MetaSpec::Colon,
                         _ => return Err(CompilationError::InvalidMetaPredicateDecl),
                     };