how to create a recombining forest tree

Question

So, I essentially want the e and f to be one node i.e the lines from b and c joining together

Here is what I have so far:

\documentclass{report}
\usepackage{forest} %Required for trees
\usepackage{tikz}
\begin{document}
\forestset{

L2/.style={draw=black,},
  L1/.style={,edge={,line width=0.8pt}},
}
\tikzset{%
  L1/.style={draw=black},
  L2/.style={line width=0.8pt, line cap=round},
}%
\begin{forest}
for tree={
    grow'=0,
    parent anchor=children,
    child anchor=parent,
    edge+=L2,
    outer sep=+1pt,
    rounded corners,
    minimum width=15mm,
    minimum height=8mm,
    l sep' =30mm,
    s sep' =20mm
  }
 [a, L1
 [b, L2
 [d, L2]
 [e, L2]
 ]
 [c, L2
 [f, L2]
 [g, L2]
 ]
 ]
 \end{forest}
\end{document}