diff --git a/lslopt/lslparse.py b/lslopt/lslparse.py
index 823a346..b4c89e4 100644
--- a/lslopt/lslparse.py
+++ b/lslopt/lslparse.py
@@ -2668,13 +2668,19 @@ list lazy_list_set(list L, integer i, list v)
 
         return self.parse(script, options)
 
-    def __init__(self, builtins = 'builtins.txt', seftable = 'seftable.txt'):
+    def __init__(self, builtins = None, seftable = None):
         """Reads the library."""
 
         self.events = {}
         self.constants = {}
         self.funclibrary = {}
 
+        if builtins is None:
+            builtins = lslcommon.DataPath + 'builtins.txt'
+
+        if seftable is None:
+            seftable = lslcommon.DataPath + 'seftable.txt'
+
         self.parse_directive_re = None
 
         # Library read code
@@ -2694,7 +2700,7 @@ list lazy_list_set(list L, integer i, list v)
         parse_num_re = re.compile(r'^\s*(-?(?=[0-9]|\.[0-9])[0-9]*((?:\.[0-9]*)?(?:[Ee][+-]?[0-9]+)?))\s*$')
         parse_str_re = re.compile(ur'^"((?:[^"\\]|\\.)*)"$')
 
-        f = open(lslcommon.DataPath + builtins, 'rb')
+        f = open(builtins, 'rb')
         try:
             linenum = 0
             try:
@@ -2850,7 +2856,7 @@ list lazy_list_set(list L, integer i, list v)
         #       that includes domain data (min, max) and possibly input
         #       parameter transformations e.g.
         #           llSensor(..., PI, ...) -> llSensor(..., 4, ...).
-        f = open(lslcommon.DataPath + seftable, 'rb')
+        f = open(seftable, 'rb')
         try:
             while True:
                 line = f.readline()
diff --git a/main.py b/main.py
index 2884afd..b39e649 100755
--- a/main.py
+++ b/main.py
@@ -172,6 +172,8 @@ Usage: {progname}
     [-h|--help]                 print this help
     [--version]                 print this program's version
     [-o|--output=<filename>]    output to file rather than stdout
+    [-b|--builtins=<filename>]  use a builtins file other than builtins.txt
+    [-S|--seftable=<filename>]  use a SEF table file other than seftable.txt
     [-H|--header]               add the script as a comment in Firestorm format
     [-T|--timestamp]            add a timestamp as a comment at the beginning
     [-y|--python-exceptions]    when an exception is raised, show a stack trace
@@ -343,11 +345,12 @@ def main(argv):
         % (b"', '".join(options - validoptions)).decode('utf8'))
 
     try:
-        opts, args = getopt.gnu_getopt(argv[1:], 'hO:o:p:P:HTy',
+        opts, args = getopt.gnu_getopt(argv[1:], 'hO:o:p:P:HTyb:S:',
             ('optimizer-options=', 'help', 'version', 'output=', 'header',
             'timestamp','python-exceptions',
             'preproc=', 'precmd=', 'prearg=', 'prenodef', 'preshow',
-            'avid=', 'avname=', 'assetid=', 'shortname='))
+            'avid=', 'avname=', 'assetid=', 'shortname=', 'builtins='
+            'seftable='))
     except getopt.GetoptError as e:
         Usage(argv[0])
         sys.stderr.write(u"\nError: " + str(e).decode('utf8') + u"\n")
@@ -366,6 +369,8 @@ def main(argv):
     mcpp_mode = False
     preshow = False
     raise_exception = False
+    builtins = None
+    seftable = None
 
     for opt, arg in opts:
         if type(opt) is unicode:
@@ -409,6 +414,12 @@ def main(argv):
         elif opt in ('-o', '--output'):
             outfile = arg
 
+        elif opt in ('-b', '--builtins'):
+            builtins = arg
+
+        elif opt in ('-S', '--seftable'):
+            seftable = arg
+
         elif opt in ('-y', '--python-exceptions'):
             raise_exception = True
 
@@ -597,7 +608,7 @@ def main(argv):
 
         if not preshow:
 
-            p = parser()
+            p = parser(builtins, seftable)
             try:
                 ts = p.parse(script, options)
             except EParse as e: