“The Blocks World”

Definitions of Descriptors:

ontable(x): block x is on top of the table

on(x,y): block x is on top of block y

clear(x): there is nothing on top of block x; therefore it can be picked up

handempty: you are not holding any block

Definitions of Operators:

Op{ACTION: pickup(x)

PRECOND: ontable(x), clear(x), handempty

EFFECT: holding(x), ~ontable(x), ~clear(x), ~handempty }

Op{ACTION: putdown(x)

PRECOND: holding(x)

EFFECT: ontable(x), clear(x), handempty, ~holding(x) }

Op{ACTION: stack(x,y)

PRECOND: holding(x), clear(y)

EFFECT: on(x,y), clear(x), handempty, ~holding(x), ~clear(y) }

Op{ACTION: unstack(x,y)

PRECOND: clear(x), on(x,y), handempty

EFFECT: holding(x), clear(y), ~clear(x), ~on(x,y), ~handempty ) }