Consecutive Underscores are reserved in GLSL 1.20

[mattdesl]

Running a rather large shader through glsl-optimizer with target ES20 gives me the following error in WebGL;

gl.getShaderInfoLog() ERROR: 0:80: 'q__19' : identifiers containing two consecutive underscores (__) are reserved as possible future keywords

This is defined in the spec, see 3.3: http://www.opengl.org/registry/doc/GLSLangSpec.Full.1.20.8.pdf