Graft a subtree into the current execution.
Given
# sub0.flo
sequence
task 'a'
task 'b'
and
# sub1.flo
sequence
task 'c'
task 'd'
then
# main.flo
concurrence
graft 'sub0.flo'
graft 'sub1' # suffix can be omitted
graft 'sub0'
#
# which is thus equivalent to
#
concurrence
sequence
task 'a'
task 'b'
sequence
task 'c'
task 'd'
sequence
task 'a'
task 'b'