# Help lines only in the lower part with tcolorbox

Using the tcolorbox package to produce exercises for a textbook, I use the option invisible to let space for students for writing their solutions. Now I want to use the help lines in overlay commands only in the lower part, and not in the upper part, so get an upper part with blank background. How can I do this?

More, can I use a more general command for help lines in the lower part, which inserts only horizontal lines, and not the vertical ones?

Edit: The solution given first is not useful when the exercise breaks, where we obtain the error message: ! Package pgf Error: No shape named segmentation is known.

I give an example where I make the exercise breaked:

\documentclass{book}

\usepackage{etoolbox}
\usepackage[most]{tcolorbox}

\NewTColorBox[auto counter,number within=chapter]{exercise}{m+O{}}{%
enhanced,
breakable,
colframe=green!20!black,
colback=yellow!10!white,
coltitle=green!40!black,
fonttitle=\bfseries,
underlay={\begin{tcbclipinterior}
(interior.north west) circle (2cm);
\draw[help lines,step=5mm,yellow!80!black,shift={(interior.north west)}]
(interior.south west) grid (interior.north east);
\end{tcbclipinterior}},
title={Exercise~ \thetcbcounter:},
label={exercise:#1},
segmentation style={double=white,draw=green!20!black,double distance=1pt,solid},
lowerbox=invisible,
savelowerto=solutions/exercise-\thetcbcounter.tex,
record={\string\solution{#1}{solutions/exercise-\thetcbcounter.tex}},
#2
}

\begin{document}
\pagestyle{empty}
\tcbstartrecording
\begin{exercise}{Ex1}[coltitle=cyan!80!black]
Compute the derivative of the following function:
\begin{equation*}
f(x)=\sin((\sin x)^2)
\end{equation*}
Compute the derivative of the following function:
\begin{equation*}
f(x)=\sin((\sin x)^2)
\end{equation*}
Compute the derivative of the following function:
\begin{equation*}
f(x)=\sin((\sin x)^2)
\end{equation*}
Compute the derivative of the following function:
\begin{equation*}
f(x)=\sin((\sin x)^2)
\end{equation*}
Compute the derivative of the following function:
\begin{equation*}
f(x)=\sin((\sin x)^2)
\end{equation*}
Compute the derivative of the following function:
\begin{equation*}
f(x)=\sin((\sin x)^2)
\end{equation*}
Compute the derivative of the following function:
\begin{equation*}
f(x)=\sin((\sin x)^2)
\end{equation*}
Compute the derivative of the following function:
\begin{equation*}
f(x)=\sin((\sin x)^2)
\end{equation*}
Compute the derivative of the following function:
\begin{equation*}
f(x)=\sin((\sin x)^2)
\end{equation*}
Compute the derivative of the following function:
\begin{equation*}
f(x)=\sin((\sin x)^2)
\end{equation*}
Compute the derivative of the following function:
\begin{equation*}
f(x)=\sin((\sin x)^2)
\end{equation*}
Compute the derivative of the following function:
\begin{equation*}
f(x)=\sin((\sin x)^2)
\end{equation*}
Compute the derivative of the following function:
\begin{equation*}
f(x)=\sin((\sin x)^2)
\end{equation*}
Compute the derivative of the following function:
\begin{equation*}
f(x)=\sin((\sin x)^2)
\end{equation*}
Compute the derivative of the following function:
\begin{equation*}
f(x)=\sin((\sin x)^2)
\end{equation*}
Compute the derivative of the following function:
\begin{equation*}
f(x)=\sin((\sin x)^2)
\end{equation*}
\tcblower
The derivative is:
$f^\prime(x) = \cos((\sin x)^2) 2\sin x \cos x.$
\end{exercise}

\tcbstoprecording
\end{document}

and its compilation:

Just replace (interior.south west) grid (interior.north east) by (interior.south west) grid (segmentation.east)

reedited as follows:

1. a new command named \myunderlay is defined
2. two tcolorbox options: underlay unbroken=\myunderlay and underlay last=\myunderlay are added in place of underlay={...}

\documentclass{book}

\usepackage{etoolbox,lipsum}
\usepackage[most]{tcolorbox}

\newcommand{\myunderlay}{
\begin{tcbclipinterior}
(interior.north west) circle (2cm);
\draw[help lines,step=5mm,yellow!80!black,shift={(interior.north west)}]
(interior.south west) grid (segmentation.east); % modified
\end{tcbclipinterior}
}

\NewTColorBox[auto counter,number within=chapter]{exercise}{m+O{}}{%
enhanced,
breakable,
colframe=green!20!black,
colback=yellow!10!white,
coltitle=green!40!black,
fonttitle=\bfseries,
underlay unbroken=\myunderlay,
underlay last=\myunderlay,
title={Exercise~ \thetcbcounter:},
label={exercise:#1},
segmentation style={double=white,draw=green!20!black,double distance=1pt,solid},
lowerbox=invisible,
%savelowerto=solutions/exercise-\thetcbcounter.tex,
record={\string\solution{#1}{solutions/exercise-\thetcbcounter.tex}},
#2,
}

\begin{document}
\pagestyle{empty}
\tcbstartrecording
\begin{exercise}{Ex1}[coltitle=cyan!80!black]
Compute the derivative of the following function:
\begin{equation*}
f(x)=\sin((\sin x)^2)
\end{equation*}

\lipsum[1-3]
\tcblower
The derivative is:
$f^\prime(x) = \cos((\sin x)^2) 2\sin x \cos x.$
\end{exercise}

\tcbstoprecording

\end{document}

• Sorry I'm confused, but this solution is not useful when the exercise breaks, and then it gives the following error: ! Package pgf Error: No shape named segmentation is known – Faouzi Bellalouna Sep 26 '18 at 12:43
• I think it maybe very helpful if you provide a reproducible example with the issue encountered! – Hafid Boukhoulda Sep 26 '18 at 12:51
• Now with the edition of @PhelypeOleinik all is good. Many thanks for the two – Faouzi Bellalouna Sep 26 '18 at 14:01
• It seems to me that there is some kind of confusion since it's @hafidBoukhoulda who reedited the answer to address the break's issue! – Hafid Boukhoulda Sep 26 '18 at 15:02