diff --git a/nim/circom_witnessgen/export_wtns.nim b/nim/circom_witnessgen/export_wtns.nim
new file mode 100644
index 0000000..ebe7512
--- /dev/null
+++ b/nim/circom_witnessgen/export_wtns.nim
@@ -0,0 +1,48 @@
+
+import std/streams
+
+import pkg/constantine/math/io/io_bigints
+
+import ./field
+
+#-------------------------------------------------------------------------------
+
+proc writeBigInt(s: Stream, big: B) = 
+  var bytes : seq[byte] = newSeq[byte](32)
+  marshal(bytes, big, littleEndian)
+  # s.write(bytes)                    # ??!?!! go home Nim, you are drunk
+  for b in bytes: s.write(b)
+
+proc writeFelt(s: Stream, x: F) =
+  s.writeBigInt(fToBig(x))
+
+#-------------------------------------------------------------------------------
+
+proc writeHeader(s: Stream, witnessLen: int) = 
+
+  # global header
+  s.write("wtns")                    # magic word "wtns"
+  s.write(uint32(2))                 # version
+  s.write(uint32(2))                 # number of sections
+
+  # section 1
+  s.write(uint32(1)   )              # section id
+  s.write(uint64(0x28))              # section length
+  s.write(uint32(32)  )              # 32 bytes per field element
+  s.writeBigInt(fieldPrime)          # the field prime
+  s.write(uint32(witnessLen))        # number of witness elements
+
+  let nbytes: uint64 = 32 * uint64(witnessLen)
+  # section 2
+  s.write(uint32(2)  )               # section id
+  s.write(nbytes)                    # section length
+
+#-------------------------------------------------------------------------------
+
+proc exportWitness*(filepath: string, witness: seq[F]) = 
+  var stream = newFileStream(filepath, fmWrite)
+  stream.writeHeader(witness.len) 
+  for i in 0..<witness.len:
+    stream.writeFelt( witness[i] )
+
+#-------------------------------------------------------------------------------
diff --git a/nim/main.nim b/nim/main.nim
index 7e5aa7f..d96546d 100644
--- a/nim/main.nim
+++ b/nim/main.nim
@@ -1,16 +1,30 @@
 
+import std/sequtils
+
+import circom_witnessgen/field
 import circom_witnessgen/graph
 import circom_witnessgen/load
 import circom_witnessgen/input_json
+import circom_witnessgen/export_wtns
+
+#-------------------------------------------------------------------------------
 
 const graph_file: string = "../tmp/graph4.bin"
 const input_file: string = "../tmp/input4.json"
+const wtns_file:  string = "../tmp/nim4.wtns"
+
+#-------------------------------------------------------------------------------
 
 when isMainModule:
+
   echo "\nloading in " & input_file
   let inp = loadInputJSON(input_file) 
-  printInputs(inp)
+  # printInputs(inp)
 
   echo "\nloading in " & graph_file
   let gr = loadGraph(graph_file)
   # echo $gr
+
+  let us: seq[int] = @[1,2,3,4,5,6,7]
+  let wtns: seq[F] = us.map(intToF);
+  exportWitness(wtns_file, wtns)
\ No newline at end of file