I'd like to set maxspeed to the minimum of maxspeed, maxspeed:forward, and maxspeed:backward.
In my lines file, I use:

maxspeed:forward=* & maxspeed!=* { set maxspeed='${maxspeed:forward}' }
maxspeed:forward=* & (maxspeed:forward<maxspeed) { set maxspeed='${maxspeed:forward}' }
maxspeed:backward=* & maxspeed!=* { set maxspeed='${maxspeed:backward}' }
maxspeed:backward=* & (maxspeed:backward<maxspeed) { set maxspeed='${maxspeed:backward}' }

That works only if exactly one tag is set. But when both forward and backward are available, I get the value of forward even if backward is less. An example is way http://www.openstreetmap.org/browse/way/70594951.

I played with additional / different rules, and my conclusion is that it is NOT POSSIBLE to compare the values of two tags with each other.

For example, I tried:

highway=* & maxspeed!=* { set maxspeed=999 }
maxspeed:forward=* & (maxspeed:forward<maxspeed) { set maxspeed='${maxspeed:forward}' }
maxspeed:backward=* & (maxspeed:backward<maxspeed) { set maxspeed='${maxspeed:backward}' }

And you can guess the result: maxspeed=999, for that way with maxspeed:forward=60 and maxspeed:backward=40.

Is my conclusion correct?
And, if so, will there be a patch?