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load(abs_integrate);

conditional_integrate(a^x + b^x, x);

gives:

%if(?%and(a-1 # 0,b-1 # 0),(log(a)*b^x+a^x*log(b))/(log(a)*log(b)),

%if(?%and(a-1 # 0,b-1 = 0),%if(a-1 # 0,(log(a)*x+a^x)/log(a),2*x),

%if(?%and(a-1 = 0,b-1 # 0),

%if(b-1 # 0,(log(b)*x+b^x)/log(b),2*x),2*x)))

The second "%if" in the second line (the one testing for "a-1 # 0") is redundant, since "a-1 # 0" was already checked for in the surrounding "%if".

The same holds true for the first "%if" in the last line.