How to operate a wheelchair safely outdoors<br>Stay on even surfaces. Uneven surfaces can make any wheelchair get off balance. Even when someone is pushing a wheelchair, an uneven surface can cause harm.
A condition for the potentially unsafe operation of a wheel chair is uneven surfaces. 
( z1 condition :domain ( z2 surface :ARG1-of ( z3 even :polarity - ) ) :purpose ( z4 operate :ARG1 ( z5 chair :mod ( z6 wheel ) ) :ARG0-of ( z7 safe :polarity - :mod ( z8 potential ) ) ) )