Index: src/library/base/R/seq.R
===================================================================
--- src/library/base/R/seq.R	(revision 53680)
+++ src/library/base/R/seq.R	(working copy)
@@ -85,4 +85,6 @@
 }
 
 ## In reverence to the very first versions of R which already had sequence():
-sequence <- function(nvec) unlist(lapply(nvec, seq_len))
+# sequence <- function(nvec) unlist(lapply(nvec, seq_len))
+sequence <- function(nvec) .Call( "sequence", as.integer(nvec), PACKAGE = "base" )
+
Index: src/main/registration.c
===================================================================
--- src/main/registration.c	(revision 53680)
+++ src/main/registration.c	(working copy)
@@ -245,6 +245,8 @@
     CALLDEF(bitwiseOr, 2),
     CALLDEF(bitwiseXor, 2),
 
+    /* sequence */
+    CALLDEF(sequence,1),
     {NULL, NULL, 0}
 };
 
Index: src/main/seq.c
===================================================================
--- src/main/seq.c	(revision 53680)
+++ src/main/seq.c	(working copy)
@@ -679,3 +679,28 @@
 
     return ans;
 }
+
+SEXP attribute_hidden sequence(SEXP x)
+{
+   R_len_t n = length(x), s = 0 ;
+   int *px = INTEGER(x) ;
+   int x_i ;
+   /* error checking */
+   for( int i=0; i<n; i++){
+       x_i = px[i] ;
+       if( x_i == NA_INTEGER || x_i <= 0 ) 
+           error( _("argument must be coercible to non-negative integer") ) ;
+       s += x_i ;
+   }
+   
+   SEXP res = PROTECT( allocVector( INTSXP, s ) ) ;
+   int *p_res = INTEGER(res) ;
+   for( int i=0; i<n; i++){
+       x_i = px[i] ;
+       for( int j=0; j<x_i; j++, p_res++)
+           *p_res = j+1 ;
+   }
+   UNPROTECT(1) ;
+   return res ;
+}
+
Index: src/main/basedecl.h
===================================================================
--- src/main/basedecl.h	(revision 53680)
+++ src/main/basedecl.h	(working copy)
@@ -114,3 +114,6 @@
 SEXP bitwiseAnd(SEXP, SEXP);
 SEXP bitwiseOr(SEXP, SEXP);
 SEXP bitwiseXor(SEXP, SEXP);
+
+SEXP sequence(SEXP);
+