1 | initial version |

In Sage, there is a argument called `alpha`

which allows to define transparency. For example, it is documented in `line2d?`

and in `text?`

which says "* alpha - A float (0.0 transparent through 1.0 opaque)*". Unfortunately,

`contour_plot`

does no allow to use `alpha`

as input.You may use `region_plot`

instead which allows to use the argument `alpha`

for transparency (0=transparent, 1=opaque):

```
sage: x,y = var('x,y')
sage: A = region_plot(x^2 + y^2 < 1, (x,-1,2), (y,-1,2), incol='blue', alpha=.4)
sage: B = region_plot((x-1)^2 + y^2 < 1, (x,-1,2), (y,-1,2), incol='red', alpha=.4)
sage: C = region_plot((x-.5)^2 + (y-.86)^2 < 1, (x,-1,2), (y,-1,2), incol='green', alpha=.4)
sage: A+B+C
```

Note that the color output is not commutative (!):

```
sage: C+B+A
```

2 | No.2 Revision |

In Sage, there is a argument called `alpha`

which allows to define transparency. For example, it is documented in `line2d?`

and in `text?`

which says "* alpha - A float (0.0 transparent through 1.0 opaque)*". Unfortunately,

`contour_plot`

does no allow to use `alpha`

as input.You may use `region_plot`

instead which allows to use the argument `alpha`

~~for transparency (0=transparent, 1=opaque):~~even if not documented:

```
sage: x,y = var('x,y')
sage: A = region_plot(x^2 + y^2 < 1, (x,-1,2), (y,-1,2), incol='blue', alpha=.4)
sage: B = region_plot((x-1)^2 + y^2 < 1, (x,-1,2), (y,-1,2), incol='red', alpha=.4)
sage: C = region_plot((x-.5)^2 + (y-.86)^2 < 1, (x,-1,2), (y,-1,2), incol='green', alpha=.4)
sage: A+B+C
```

Note that the color output is not commutative (!):

```
sage: C+B+A
```

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