OB10-023 Parse prover output with presence of mk_t__ref.

Enabling the transformation detect_polymorphism caused some variables be
of reference type and their values outputed in the form "mk_t__ref value".
This was not recognized by the counterexample model parser.

* src/driver/parse_smtv2_model_lexer.mll
Add new token corresponding to string "mk_t__ref"

* src/driver/parse_smtv2_model_parser.mly
Value can be also of the form "(mk_t__ref value)"

src/driver/parse_smtv2_model_lexer.mll

@@ -13,6 +13,7 @@ rule token = parse
     { token lexbuf }
   | space+ as space_str
       { SPACE (space_str) }
+  | "mk_t__ref" { MK_T_REF }
   | "store" { STORE }
   | "const" { CONST }
   | "as" { AS }

src/driver/parse_smtv2_model_parser.mly

@@ -13,6 +13,7 @@
 %token <string * string> DEC_STR
 %token <string * string> MINUS_DEC_STR
 %token LPAREN RPAREN
+%token MK_T_REF
 %token EOF
 %%
 
@@ -53,6 +54,7 @@ text_without_int:
 | AS { "as" }
 
 value:
+| LPAREN MK_T_REF SPACE value RPAREN { $4 }
 | integer { $1 }
 | decimal { $1 }
 | other_val_str { Model_parser.Unparsed $1 }