public Cell<C> space (float top, float left, float bottom, float right) {
if (top < 0) throw new IllegalArgumentException("top cannot be < 0.");
if (left < 0) throw new IllegalArgumentException("left cannot be < 0.");
if (bottom < 0) throw new IllegalArgumentException("bottom cannot be < 0.");
if (right < 0) throw new IllegalArgumentException("right cannot be < 0.");
spaceTop = new FixedValue(top);
spaceLeft = new FixedValue(left);
spaceBottom = new FixedValue(bottom);
spaceRight = new FixedValue(right);
return this;
}