diff --git a/ch13.jl b/ch13.jl
index fb388a4..37e6325 100644
--- a/ch13.jl
+++ b/ch13.jl
@@ -1,3 +1,9 @@
+# Bogumił Kamiński, 2022
+
+# Codes for chapter 13
+
+# Codes for section 13.1
+
 using CSV
 using CategoricalArrays
 using DataFrames
@@ -38,12 +44,37 @@ close(archive)
 summary(owensboro)
 describe(owensboro, :nunique, :nmissing, :eltype)
 
+# Code for listing 13.1
+
 select!(owensboro, :date, :type, :arrest_made, :violation);
 summary(owensboro)
 describe(owensboro, :nunique, :nmissing, :eltype)
 
+# Code for listing 13.2
+
+df = DataFrame(id=[1, 2, 1, 2], v=1:4)
+combine(df, :v => sum => :sum)
+transform(df, :v => sum => :sum)
+select(df, :v => sum => :sum)
+gdf = groupby(df, :id)
+combine(gdf, :v => sum => :sum)
+transform(gdf, :v => sum => :sum)
+select(gdf, :v => sum => :sum)
+
+# Code for listing 13.3
+
+select(df,
+    :v => identity => :v1,
+    :v => identity,
+    :v => :v2,
+    :v)
+
+# Codes for section 13.2
+
 owensboro.violation
 
+# Code for listing 13.4
+
 violation_list = [strip.(split(x, ";"))
                   for x in owensboro.violation]
 violation_flat = reduce(vcat, violation_list)
@@ -51,6 +82,8 @@ violation_flat_clean = [contains(x, "SPEEDING") ?
                         "SPEEDING" : x for x in violation_flat]
 sort(freqtable(violation_flat_clean), rev=true)
 
+# Code for listing 13.5
+
 agg_violation = @chain owensboro begin
     select(:violation =>
            ByRow(x -> strip.(split(x, ";"))) =>
@@ -64,27 +97,39 @@ agg_violation = @chain owensboro begin
     sort(:count, rev=true)
 end
 
+# Code explaining ByRow
+
 sqrt(4)
 sqrt([4, 9, 16])
 ByRow(sqrt)([4, 9, 16])
 f = ByRow(sqrt)
 f([4, 9, 16])
 
+# Code explaining flatten
+
 df = DataFrame(id=1:2, v=[[11, 12], [13, 14, 15]])
 flatten(df, :v)
 
+# Code explaining nrow
+
 @chain DataFrame(id=[1, 1, 2, 2, 2]) begin
     groupby(:id)
     combine(nrow, nrow => :rows)
 end
 
+# Code explaining sort
+
 df = DataFrame(a=[2, 1, 2, 1, 2], b=5:-1:1)
 sort(df, :b)
 sort(df, [:a, :b])
 
+# Code showing an example transformation
+
 df = DataFrame(x=[4, 9, 16])
 transform(df, :x => ByRow(sqrt))
 
+# Code showing usage of DataFramesMeta.jl
+
 @chain owensboro begin
     @rselect(:v=strip.(split(:violation, ";")))
     flatten(:v)
@@ -95,11 +140,17 @@ transform(df, :x => ByRow(sqrt))
     sort(:count, rev=true)
 end
 
+# Code showing comparison of DataFramesMeta.jl vs DataFrames.jl
+
 df = DataFrame(x=[4, 9, 16])
 @select(df, :s = sqrt.(:x))
 @rselect(df, :s = sqrt(:x))
 select(df, :x => ByRow(sqrt) => :s)
 
+# Codes for section 13.3
+
+# Code for listing 13.6
+
 owensboro2 = select(owensboro,
     :arrest_made => :arrest,
     :date => ByRow(dayofweek) => :day,
@@ -108,6 +159,8 @@ owensboro2 = select(owensboro,
      ByRow(x -> contains(x, agg_violation.v[i])) =>
      "v$i" for i in 1:4])
 
+# Code explaining programmatic generation of transformations
+
 [:violation =>
  ByRow(x -> contains(x, agg_violation.v[i])) =>
  "v$i" for i in 1:4]
@@ -115,42 +168,74 @@ owensboro2 = select(owensboro,
 combine(owensboro, [:date :arrest_made] .=> [minimum, maximum])
 [:date :arrest_made] .=> [minimum, maximum]
 
+# Code for listing 13.7
+
 weekdays = DataFrame(day=1:7,
                      dayname=categorical(dayname.(1:7); ordered=true))
 isordered(weekdays.dayname)
 levels(weekdays.dayname)
 levels!(weekdays.dayname, weekdays.dayname)
 
+# Code showing join operation
+
 leftjoin!(owensboro2, weekdays; on=:day)
 
+# Code for listing 13.8
+
 @chain owensboro2 begin
     groupby([:day, :dayname]; sort=true)
     combine(nrow)
 end
 
+# Code showing creation of frequency table from a data frame
+
 freqtable(owensboro2, :dayname, :day)
 
+# Code for listing 13.9
+
 @chain owensboro2 begin
     groupby([:day, :dayname]; sort=true)
     combine(nrow)
     unstack(:dayname, :day, :nrow; fill=0)
 end
 
+# Code for dropping rows with missing data
+
 dropmissing!(owensboro2)
+
+# Code for dropping unwanted column
+
 select!(owensboro2, Not(:day))
 
+# Codes for section 13.4
+
+# Code for listing 13.10
+
 Random.seed!(1234);
 owensboro2.train = rand(Bernoulli(0.7), nrow(owensboro2));
 mean(owensboro2.train)
 
+# Code for listing 13.11
+
 train = subset(owensboro2, :train)
 test = subset(owensboro2, :train => ByRow(!))
 
+# Code showing subsetting with DataFramesMeta.jl
+
+@rsubset(owensboro2, !(:train))
+
+# Code building a predictive model
+
 model = glm(@formula(arrest~dayname+type+v1+v2+v3+v4),
             train, Binomial(), LogitLink())
+
+# Code for making predictions
+
 train.predict = predict(model)
 test.predict = predict(model, test)
 
+# Code for producing histograms showing predictions
+
 test_groups = groupby(test, :arrest);
 histogram(test_groups[(false,)].predict;
           bins=10, normalize=:probability,
@@ -159,6 +244,8 @@ histogram!(test_groups[(true,)].predict;
            bins=10, normalize=:probability,
            fillalpha=0.5, label="true")
 
+# Code for listing 13.12
+
 test_roc =  roc(test, score=:predict, target=:arrest)
 plot(test_roc.pfa, 1 .- test_roc.pmiss;
      legend=:bottomright,