// operand access
size_t nops() const { return bp->nops(); }
ex op(size_t i) const { return bp->op(i); }
- ex operator[](const ex & index) const { return (*bp)[index]; }
- ex operator[](size_t i) const { return (*bp)[i]; }
+ ex operator[](const ex & index) const { return (const_cast<const basic&>(*bp))[index]; }
+ ex operator[](size_t i) const { return (const_cast<const basic&>(*bp))[i]; }
ex & let_op(size_t i);
ex & operator[](const ex & index);
ex & operator[](size_t i);