diff --git a/src/blob/mod.rs b/src/blob/mod.rs
index daafcf1c8..76f28fef2 100644
--- a/src/blob/mod.rs
+++ b/src/blob/mod.rs
@@ -2,7 +2,7 @@ mod ops;
 pub mod positive;
 pub mod negative;
 
-pub use self::positive::{PositiveBlob, RawPositiveBlobBuilder, PositiveBlobBuilder};
+pub use self::positive::{PositiveBlob, PositiveBlobBuilder};
 pub use self::negative::{NegativeBlob, NegativeBlobBuilder};
 pub use self::ops::OpBuilder;
 
diff --git a/src/blob/ops.rs b/src/blob/ops.rs
index 90f91547f..0a9da3abf 100644
--- a/src/blob/ops.rs
+++ b/src/blob/ops.rs
@@ -6,7 +6,7 @@ use itertools::{Itertools, Either};
 use sdset::duo::DifferenceByKey;
 use sdset::{Set, SetOperation};
 
-use crate::blob::{Blob, Sign, PositiveBlob, RawPositiveBlobBuilder, NegativeBlob};
+use crate::blob::{Blob, Sign, PositiveBlob, PositiveBlobBuilder, NegativeBlob};
 use crate::blob::{positive, negative};
 
 fn blob_same_sign(a: &Blob, b: &Blob) -> bool {
@@ -56,7 +56,7 @@ impl OpBuilder {
                     }
 
                     let mut stream = op_builder.union().into_stream();
-                    let mut builder = RawPositiveBlobBuilder::memory();
+                    let mut builder = PositiveBlobBuilder::memory();
                     while let Some((input, doc_indexes)) = stream.next() {
                         // FIXME empty doc_indexes must be handled by OpBuilder
                         if !doc_indexes.is_empty() {
@@ -81,7 +81,7 @@ impl OpBuilder {
         let mut zipped = positives.into_iter().zip(negatives);
         let mut buffer = Vec::new();
         zipped.try_fold(PositiveBlob::default(), |base, (positive, negative)| {
-            let mut builder = RawPositiveBlobBuilder::memory();
+            let mut builder = PositiveBlobBuilder::memory();
             let doc_ids = Set::new_unchecked(negative.as_ref());
 
             let op_builder = positive::OpBuilder::new().add(&base).add(&positive);
diff --git a/src/blob/positive/blob.rs b/src/blob/positive/blob.rs
index 7cbcf47ab..ee143dbef 100644
--- a/src/blob/positive/blob.rs
+++ b/src/blob/positive/blob.rs
@@ -6,7 +6,7 @@ use std::error::Error;
 use fst::{map, Map, Streamer, IntoStreamer};
 
 use crate::DocIndex;
-use crate::data::{DocIndexes, RawDocIndexesBuilder, DocIndexesBuilder};
+use crate::data::{DocIndexes, RawDocIndexesBuilder};
 use serde::ser::{Serialize, Serializer, SerializeTuple};
 use serde::de::{self, Deserialize, Deserializer, SeqAccess, Visitor};
 
@@ -133,15 +133,15 @@ impl<'de> Deserialize<'de> for PositiveBlob {
     }
 }
 
-pub struct RawPositiveBlobBuilder<W, X> {
+pub struct PositiveBlobBuilder<W, X> {
     map: fst::MapBuilder<W>,
     indexes: RawDocIndexesBuilder<X>,
     value: u64,
 }
 
-impl RawPositiveBlobBuilder<Vec<u8>, Vec<u8>> {
+impl PositiveBlobBuilder<Vec<u8>, Vec<u8>> {
     pub fn memory() -> Self {
-        RawPositiveBlobBuilder {
+        PositiveBlobBuilder {
             map: fst::MapBuilder::memory(),
             indexes: RawDocIndexesBuilder::memory(),
             value: 0,
@@ -149,15 +149,18 @@ impl RawPositiveBlobBuilder<Vec<u8>, Vec<u8>> {
     }
 }
 
-impl<W: Write, X: Write> RawPositiveBlobBuilder<W, X> {
+impl<W: Write, X: Write> PositiveBlobBuilder<W, X> {
     pub fn new(map: W, indexes: X) -> Result<Self, Box<Error>> {
-        Ok(RawPositiveBlobBuilder {
+        Ok(PositiveBlobBuilder {
             map: fst::MapBuilder::new(map)?,
             indexes: RawDocIndexesBuilder::new(indexes),
             value: 0,
         })
     }
 
+    /// If a key is inserted that is less than or equal to any previous key added,
+    /// then an error is returned. Similarly, if there was a problem writing
+    /// to the underlying writer, an error is returned.
     // FIXME what if one write doesn't work but the other do ?
     pub fn insert(&mut self, key: &[u8], doc_indexes: &[DocIndex]) -> Result<(), Box<Error>> {
         self.map.insert(key, self.value)?;
@@ -176,48 +179,3 @@ impl<W: Write, X: Write> RawPositiveBlobBuilder<W, X> {
         Ok((map, indexes))
     }
 }
-
-pub struct PositiveBlobBuilder<W, X> {
-    map: W,
-    indexes: DocIndexesBuilder<X>,
-}
-
-impl<W: Write, X: Write> PositiveBlobBuilder<W, X> {
-    pub fn new(map: W, indexes: X) -> Self {
-        Self { map, indexes: DocIndexesBuilder::new(indexes) }
-    }
-
-    pub fn insert<S: Into<String>>(&mut self, key: S, index: DocIndex) {
-        self.indexes.insert(key.into(), index)
-    }
-
-    pub fn finish(self) -> Result<(), Box<Error>> {
-        self.into_inner().map(drop)
-    }
-
-    pub fn into_inner(self) -> Result<(W, X), Box<Error>> {
-        // FIXME insert a magic number that indicates if the endianess
-        //       of the input is the same as the machine that is reading it.
-
-        let map = {
-            let mut keys_builder = fst::MapBuilder::new(self.map)?;
-            let keys = self.indexes.keys().map(|(s, v)| (s, *v));
-            keys_builder.extend_iter(keys)?;
-            keys_builder.into_inner()?
-        };
-
-        let indexes = self.indexes.into_inner()?;
-
-        Ok((map, indexes))
-    }
-}
-
-impl PositiveBlobBuilder<Vec<u8>, Vec<u8>> {
-    pub fn memory() -> Self {
-        PositiveBlobBuilder::new(Vec::new(), Vec::new())
-    }
-
-    pub fn build(self) -> Result<PositiveBlob, Box<Error>> {
-        self.into_inner().and_then(|(m, i)| PositiveBlob::from_bytes(m, i))
-    }
-}
diff --git a/src/blob/positive/mod.rs b/src/blob/positive/mod.rs
index be895b80b..d8e4e164e 100644
--- a/src/blob/positive/mod.rs
+++ b/src/blob/positive/mod.rs
@@ -1,5 +1,5 @@
 mod blob;
 mod ops;
 
-pub use self::blob::{PositiveBlob, RawPositiveBlobBuilder, PositiveBlobBuilder};
+pub use self::blob::{PositiveBlob, PositiveBlobBuilder};
 pub use self::ops::OpBuilder;
diff --git a/src/index/update/negative_update.rs b/src/index/update/negative_update.rs
index 6c82919b5..342fde98c 100644
--- a/src/index/update/negative_update.rs
+++ b/src/index/update/negative_update.rs
@@ -50,7 +50,7 @@ impl NegativeUpdateBuilder {
         let doc_ids = DocIds::from_bytes(bytes)?;
         let blob = Blob::Negative(NegativeBlob::from_raw(doc_ids));
         let bytes = bincode::serialize(&blob)?;
-        file_writer.merge(DATA_INDEX, &bytes);
+        file_writer.merge(DATA_INDEX, &bytes)?;
 
         // FIXME remove this ugly thing !
         // let Blob::Negative(negative_blob) = blob;