diff --git a/lark/lexer.py b/lark/lexer.py
index 730d95e..72b299c 100644
--- a/lark/lexer.py
+++ b/lark/lexer.py
@@ -307,7 +307,8 @@ class TraditionalLexer(Lexer):
                 if t.pattern.min_width == 0:
                     raise LexError("Lexer does not allow zero-width terminals. (%s: %s)" % (t.name, t.pattern))
 
-            assert set(conf.ignore) <= {t.name for t in terminals}
+            if not (set(conf.ignore) <= {t.name for t in terminals}):
+                raise LexError("Ignore terminals are not defined: %s" % (set(conf.ignore) - {t.name for t in terminals}))
 
         # Init
         self.newline_types = frozenset(t.name for t in terminals if _regexp_has_newline(t.pattern.to_regexp()))
diff --git a/lark/load_grammar.py b/lark/load_grammar.py
index a07769f..f7da69d 100644
--- a/lark/load_grammar.py
+++ b/lark/load_grammar.py
@@ -93,6 +93,7 @@ TERMINALS = {
     'COMMENT': r'\s*//[^\n]*',
     '_TO': '->',
     '_IGNORE': r'%ignore',
+    '_OVERRIDE': r'%override',
     '_DECLARE': r'%declare',
     '_IMPORT': r'%import',
     'NUMBER': r'[+-]?\d+',
@@ -148,7 +149,8 @@ RULES = {
 
     'term': ['TERMINAL _COLON expansions _NL',
              'TERMINAL _DOT NUMBER _COLON expansions _NL'],
-    'statement': ['ignore', 'import', 'declare'],
+    'statement': ['ignore', 'import', 'declare', 'override_rule'],
+    'override_rule': ['_OVERRIDE rule'],
     'ignore': ['_IGNORE expansions _NL'],
     'declare': ['_DECLARE _declare_args _NL'],
     'import': ['_IMPORT _import_path _NL',
@@ -949,6 +951,7 @@ class GrammarLoader:
 
         # Execute statements
         ignore, imports = [], {}
+        overriding_rules = []
         for (stmt,) in statements:
             if stmt.data == 'ignore':
                 t ,= stmt.children
@@ -997,6 +1000,9 @@ class GrammarLoader:
             elif stmt.data == 'declare':
                 for t in stmt.children:
                     term_defs.append([t.value, (None, None)])
+            elif stmt.data == 'override_rule':
+                r ,= stmt.children
+                overriding_rules.append(options_from_rule(*r.children))
             else:
                 assert False, stmt
 
@@ -1009,6 +1015,15 @@ class GrammarLoader:
             term_defs += new_td
             rule_defs += new_rd
 
+        for r in overriding_rules:
+            name = r[0]
+            overridden, rule_defs = classify_bool(rule_defs, lambda r: r[0] == name)    # FIXME inefficient
+            if not overridden:
+                raise GrammarError("Cannot override a nonexisting rule: %s" % name)
+            rule_defs.append(r)
+
+        ## Handle terminals
+
         # Verify correctness 1
         for name, _ in term_defs:
             if name.startswith('__'):
@@ -1045,10 +1060,10 @@ class GrammarLoader:
 
         resolve_term_references(term_defs)
 
-        rules = rule_defs
+        ## Handle rules
 
         rule_names = {}
-        for name, params, _x, option in rules:
+        for name, params, _x, option in rule_defs:
             # We can't just simply not throw away the tokens later, we need option.keep_all_tokens to correctly generate maybe_placeholders
             if self.global_keep_all_tokens:
                 option.keep_all_tokens = True
@@ -1059,7 +1074,7 @@ class GrammarLoader:
                 raise GrammarError("Rule '%s' defined more than once" % name)
             rule_names[name] = len(params)
 
-        for name, params , expansions, _o in rules:
+        for name, params , expansions, _o in rule_defs:
             for i, p in enumerate(params):
                 if p in rule_names:
                     raise GrammarError("Template Parameter conflicts with rule %s (in template %s)" % (p, name))
@@ -1082,7 +1097,7 @@ class GrammarLoader:
                     if sym not in rule_names and sym not in params:
                         raise GrammarError("Rule '%s' used but not defined (in rule %s)" % (sym, name))
 
-        return Grammar(rules, term_defs, ignore_names)
+        return Grammar(rule_defs, term_defs, ignore_names)
 
 
 def load_grammar(grammar, source, import_paths, global_keep_all_tokens):
diff --git a/tests/test_grammar.py b/tests/test_grammar.py
index 363f897..3ce76f6 100644
--- a/tests/test_grammar.py
+++ b/tests/test_grammar.py
@@ -21,6 +21,19 @@ class TestGrammar(TestCase):
                 else:
                     assert False, "example did not raise an error"
 
+    def test_override(self):
+        # Overrides the 'sep' template in existing grammar to add an optional terminating delimiter
+        # Thus extending it beyond its original capacity
+        p = Lark("""
+            %import .test_templates_import (start, sep)
+
+            %override sep{item, delim}: item (delim item)* delim?
+            %ignore " "
+        """)
+
+        a = p.parse('[1, 2, 3]')
+        b = p.parse('[1, 2, 3, ]')
+        assert a == b