Add constructor to 'Chessman' class;
add 'color' and 'src' attributes to 'Chessman' instances;
modify the function 'agent-at' so that is a chessman factory;
modify the function 'gs.behind' so that is a 'Pawn' method.
Adapt methods so that they make use of the color
attribute of their receiver instance
Incorporate the functions
'gs.ally?', 'gs.opponent?', and 'gs.tgt-assailed?' as methods in
the class 'Chessman'.
Adapt methods so that they make use of the position ('src')
attribute of their receiver instance