digraph G {

bgcolor=transparent;
overlap=compress;
rankdir=TB;

node[
  fontname=Monaco,
  penwidth=1,
  fontsize=20,
  margin=.1,
  shape=box,
  style="filled"
]

session [label="Session"]

file [label="File"]
case_1 [label="Case[a=1]"]
case_2 [label="Case[a=4]"]

session -> file [style=tapered, shape=rectangle]
file -> case_1 [style=tapered, shape=rectangle]
file -> case_2 [style=tapered, shape=rectangle]

}
