yes, you should leave the "+mov r0.a,c0". it copies 1 into the alpha channel of the resulting pixels and thus makes the terrain non-transparent.

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but it isn't really needed because the alpha which results from the lrp would add up to 1 anyway. i just thought the shader is easier to understand this way and i think because of the + it runs parallel to the lrp command and doesn't slow down the execution of the shader.