请帮忙解释一下代码，我看不懂
主要就是有颜色的句子不明白，请帮忙看下，谢谢了
<head>
<title>A simple calculator</title>
<script type = "text/javascript">
<!--
function doit() {
form.input.value = eval(form.input.value)
}
</script>
</head>
<body>
<form name = "form" action = "">
<table border = "1" width = "450" cellspacing = "5"
summary = "This is a calculator used to calculate simple mathematics operations">
<caption>Simple Calculator</caption>
<thead></thead>
<tbody>
<tr>
<td colspan = "3" height = "15">
<input name = "input" type = "text" size = "32"></td>
</tr>
<tr>
<td width = "60" height = "15">
<input name = "one" type = "button" value = " 1 "
onClick = "form.input.value += '1'" /></td>
<td width = "60" height = "15">
<input name = "two" type = "button" value = " 2 "
onClick = "form.input.value += '2'" /></td>
<td width = "60" height = "15">
<input name = "three" type = "button" value = " 3 "
onClick = "form.input.value += '3'" /></td>
<td width = "1" height = "15"></td>
<td width = "60" height = "15">
<input name = "MultiplySign" type = "button" value = " * "
onClick = "form.input.value += '*'" /></td>
</tr>
<tr>
<td width = "60" height = "15">
<input name = "equal" type = "button" value = " = "
onClick = "doit()" /></td>
</tr>
</table>
</form>
</body>
------解决方案--------------------form.input.value += '1'
相当于在form.input.value = form.input.value + '1'
意思是在原来的基础上加1
------解决方案--------------------将原先的value后追加(1,2,3,*)
。。。。
------解决方案--------------------这个form.input.value 指的是这个 <input name = "input" type = "text" size = "32">的value，
按钮的value值只是做显示用，显示在按钮的表面，
当然他的功能，就是onClick里面执行的，把上面那个input的值改变。
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