diff --git a/include/chibi/sexp.h b/include/chibi/sexp.h
index 8756c17d..23d7ab53 100755
--- a/include/chibi/sexp.h
+++ b/include/chibi/sexp.h
@@ -156,6 +156,7 @@ enum sexp_types {
   SEXP_CND,
   SEXP_REF,
   SEXP_SET,
+  SEXP_SET_SYN,
   SEXP_SEQ,
   SEXP_LIT,
   SEXP_STACK,
@@ -448,6 +449,9 @@ struct sexp_struct {
     struct {
       sexp var, value, source;
     } set;
+    struct {
+      sexp var, value, source;
+    } set_syn;
     struct {
       sexp name, cell, source;
     } ref;
@@ -725,6 +729,7 @@ sexp sexp_make_flonum(sexp ctx, double f);
 #define sexp_cndp(x)        (sexp_check_tag(x, SEXP_CND))
 #define sexp_refp(x)        (sexp_check_tag(x, SEXP_REF))
 #define sexp_setp(x)        (sexp_check_tag(x, SEXP_SET))
+#define sexp_set_synp(x)    (sexp_check_tag(x, SEXP_SET_SYN))
 #define sexp_seqp(x)        (sexp_check_tag(x, SEXP_SEQ))
 #define sexp_litp(x)        (sexp_check_tag(x, SEXP_LIT))
 #define sexp_contextp(x)    (sexp_check_tag(x, SEXP_CONTEXT))
@@ -1075,6 +1080,10 @@ SEXP_API sexp sexp_make_unsigned_integer(sexp ctx, sexp_luint_t x);
 #define sexp_set_value(x)     (sexp_field(x, set, SEXP_SET, value))
 #define sexp_set_source(x)    (sexp_field(x, set, SEXP_SET, source))
 
+#define sexp_set_syn_var(x)    (sexp_field(x, set, SEXP_SET_SYN, var))
+#define sexp_set_syn_value(x)  (sexp_field(x, set, SEXP_SET_SYN, value))
+#define sexp_set_syn_source(x) (sexp_field(x, set, SEXP_SET_SYN, source))
+
 #define sexp_ref_name(x)      (sexp_field(x, ref, SEXP_REF, name))
 #define sexp_ref_cell(x)      ((x)->value.ref.cell)
 #define sexp_ref_loc(x)       (sexp_cdr(sexp_ref_cell(x)))
diff --git a/sexp.c b/sexp.c
index 25d24413..630a2ee4 100644
--- a/sexp.c
+++ b/sexp.c
@@ -218,6 +218,7 @@ static struct sexp_type_struct _sexp_type_specs[] = {
   {SEXP_CND, sexp_offsetof(cnd, test), 4, 4, 0, 0, sexp_sizeof(cnd), 0, 0, 0, 0, 0, 0, 0, 0, (sexp)"If", SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, (sexp)sexp_write_simple_object, NULL, NULL},
   {SEXP_REF, sexp_offsetof(ref, name), 3, 3, 0, 0, sexp_sizeof(ref), 0, 0, 0, 0, 0, 0, 0, 0, (sexp)"Ref", SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, (sexp)sexp_write_simple_object, NULL, NULL},
   {SEXP_SET, sexp_offsetof(set, var), 3, 3, 0, 0, sexp_sizeof(set), 0, 0, 0, 0, 0, 0, 0, 0, (sexp)"Set!", SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, (sexp)sexp_write_simple_object, NULL, NULL},
+  {SEXP_SET_SYN, sexp_offsetof(set_syn, var), 3, 3, 0, 0, sexp_sizeof(set_syn), 0, 0, 0, 0, 0, 0, 0, 0, (sexp)"Set-Syn!", SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, (sexp)sexp_write_simple_object, NULL, NULL},
   {SEXP_SEQ, sexp_offsetof(seq, ls), 2, 2, 0, 0, sexp_sizeof(seq), 0, 0, 0, 0, 0, 0, 0, 0, (sexp)"Seq", SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, (sexp)sexp_write_simple_object, NULL, NULL},
   {SEXP_LIT, sexp_offsetof(lit, value), 2, 2, 0, 0, sexp_sizeof(lit), 0, 0, 0, 0, 0, 0, 0, 0, (sexp)"Lit", SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, (sexp)sexp_write_simple_object, NULL, NULL},
   {SEXP_STACK, sexp_offsetof(stack, data), 0, 0, sexp_offsetof(stack, top), 1, sexp_sizeof(stack), offsetof(struct sexp_struct, value.stack.length), sizeof(sexp), 0, 0, 0, 0, 0, 0, (sexp)"Stack", SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, SEXP_FALSE, NULL, NULL, NULL},