How to define a math constant

How to define a math constant.ipynb

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            "text/plain": "\u001b[36m  @irrational sym val def\u001b[39m\n\u001b[36m  @irrational(sym, val, def)\u001b[39m\n\n  Define a new \u001b[36mIrrational\u001b[39m value, \u001b[36msym\u001b[39m, with pre-computed \u001b[36mFloat64\u001b[39m value \u001b[36mval\u001b[39m,\n  and arbitrary-precision definition in terms of \u001b[36mBigFloat\u001b[39ms given be the\n  expression \u001b[36mdef\u001b[39m.",
            "text/markdown": "```\n@irrational sym val def\n@irrational(sym, val, def)\n```\n\nDefine a new `Irrational` value, `sym`, with pre-computed `Float64` value `val`, and arbitrary-precision definition in terms of `BigFloat`s given be the expression `def`.\n",
            "text/latex": "\\begin{verbatim}\n@irrational sym val def\n@irrational(sym, val, def)\n\\end{verbatim}\nDefine a new \\texttt{Irrational} value, \\texttt{sym}, with pre-computed \\texttt{Float64} value \\texttt{val}, and arbitrary-precision definition in terms of \\texttt{BigFloat}s given be the expression \\texttt{def}.\n\n"
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      "source": "# Base.@irrational math_constant_name value_in_Float64 how_to_calculate_in_BigFloat\nBase.@irrational 🐻 1.41421356237309504880 √big(2)\n\n\"\"\"\n    🐻\n    bear\n\nThe constant √2.\n\"\"\"\n🐻, const bear = 🐻",
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            "text/plain": "bear"
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      "source": "🐻 # \\:bear: TAB",
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          "data": {
            "text/plain": "🐻 = 1.4142135623730..."
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          "data": {
            "text/plain": "🐻 = 1.4142135623730..."
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      "source": "float(🐻)",
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      "source": "big(🐻)",
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      "source": "Float32(🐻)",
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            "text/plain": "1.4142135f0"
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