Special Effects (Original enhancement)

`SET DRAW MODE HIDDEN`
Transfers to the mode of drawing only on the internal bitmap.
`SET DRAW MODE EXPLICIT`
Reflects the internal image, and transfers to the mode of drawing both on the screen and internal bitmap.

The following Statements designate how to mix the color at the point and the drawing color.
These statements do not affect the behavior of drawing of text except PLOT LETTERS.
( Also do not affect the numerals drawn with DRAW GRID or DRAW AXES. )
★ These may not work properly. Particularly, they are of no effect on Mac.
`SET DRAW MODE OVERWRITE`
Paint the points with the color designated.
`SET DRAW MODE MASK`
subtractive mixture
`SET DRAW MODE MERGE`
additive mixture
`SET DRAW MODE XOR`
When figures are drawn twice, they shall vanish.
When the color at the point is black, the point shall be painted with the designated color.
`SET DRAW MODE NOTXOR`
When figures are drawn twice, they shall vanish.
When the color at the point is white, the point shall be painted with the designated color.