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# This file contains the functions:
# DTP_LeftOf
#############################################################################
# Input: letters left <> right which occur during a collection to the left
# Output: - true, if at the first instance that left and right both
# label generators, the generators labelled by left
# is to the left of the generator labelled by right
# - false, otherwise
DTP_LeftOf := function(left, right)
local is_atom_left, is_atom_right, earlier;
# IsBound(letter.side) <=> letter is an atom
is_atom_left := IsBound(left.side);
is_atom_right := IsBound(right.side);
if is_atom_left and is_atom_right then # both are atoms
if left.side = DT_left and right.side = DT_right then
return true;
elif left.side = DT_right and right.side = DT_left then
return false;
elif left.num = right.num then
# now both belong to the same side and label the same generator
return left.pos < right.pos;
fi;
return left.num < right.num;
fi;
# now at least one letter is a non-atom
if (not is_atom_left and not is_atom_right) # both are non-atoms
and (left.right = right.right) and (left.left = right.left) then
if left.num = right.num then
return left.pos < right.pos;
fi;
return left.num < right.num;
fi;
# Find out if left or right occurred earlier:
# "earlier" is a boolean which is true if and only if left occurred
# strictly earlier during a collection than right
if is_atom_left then
earlier := true;
elif is_atom_right then
earlier := false;
elif left.right = right.right then
earlier := DTP_LeftOf(right.left, left.left);
elif left.right.num = right.right.num then
earlier := DTP_LeftOf(left.right, right.right);
else
earlier := (left.right.num < right.right.num);
fi;
# make sure that right occurred earlier than left
if earlier then
return not DTP_LeftOf(right, left);
fi;
# now left must be a non-atom and we may access its left and right part
if right.num < left.right.num then
return false;
fi;
if right = left.right then
return false;
fi;
if right.num = left.right.num then
return DTP_LeftOf(left.right, right);
fi;
# now right.num > left.right.num
if right = left.left then
return false;
fi;
return DTP_LeftOf(left.left, right);
end;
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